Backed out changeset ddadbdf20e70, no effect on current problem.

GH CI: try reverting to previous version of ubuntu

IDE tests: ensure that file browser idled before entering path in new_project

tests/tools: move comment and remove dead code in Dockerfile

tests: add tesseract training data "tessdata", still trying to enhance OCR reliability in UI tests

tests: attemps to make OCR based hits more stable

Tests: fix path logic in Docker creation script, consequence of previous commit.

Tests: cleanup and tidy docker definitions and scripts

Merged changes from default in wxPython4 branch

SVGHMI: fix XY graph init (consequence of earlier change) + reindent

Earlier change did modify the variables attributes (options) are stored in widget objects. XY graph uses variables' min and max to determine range.

SVGHMI: move declaration of "xmlns" javascript const so that widgets can use it.

This was broken in earlier commit, when changing initial execution order because of implementing websocket reconnect.

SVGHMI: add Page assignment use case in svghmi test project.

Marged default in wxPython4

SVGHMI: update generated xslt

SVGHMI: avoid Widget's member name colision in between enable expressions and assignments.

SVGHMI: Add arbitrary variable assignment when entering Pages

Usage similar to Assign widget.
Required instanciating PageWidgets for each page, so that assigned variables can be subsribed.

SVGHMI: update generated XSLT

SVGHMI: fix periodic reconnect triggering watchdog

IDE: Optimization of modification events processing in text editors.

Too many modifications types where registered, and then too many events were fired.
Also, in case of uninterrupted sequence of events, updates to the model is deferred to the end of that sequence (wx.Callafter).

SVGHMI: cosmetic change

SVGHMI: do not try to reconnect websocket if closed properly by server.

Tests: force bigger fonts and flat theme in GTK to make OCR more reliable.

OPC-UA: tolerate better pre-encryption formated client configuration file.

OPC-UA: only support the encryption policy selected in config.

By default open62541 client accepts all supported policies, but in makes problem
when negociating with some servers while most clients seems to only support
one policy at a time.

Tests: add encrypted OPCUA browsing test.

Tests: IDE: add stdout monitoring capabilities to auxiliary process

OPC-UA: Update CTN "ChangesToSave" status when variable selection change.

- fixes changes not being saved when no other changes happening to configuration
- added CTNMarkModified to ConfigTreeNode in order to generalize

Tests: Add project for OPC-UA server browsing and variable picking UI test

Tests: Add OPC-UA server browsing and veriable picking UI test.

Tests: Add 'dragNdrop' for UI tests with slower begining than sikuli's dragDrop

Tests: UI tests now avoid polling stdout if process didn't start.

Tests: UI tests can now load projects from tests/projects

Tests: add command for UI tests to launch and close Auxiliary Process

OPC-UA: better handling and reporting of server browser's connection errors.

OPC-UA: Extend Drag'N'Drop target to whole input or output panel not only grid.

Merge default in wxPython4

SVGHMI: fix HMI freeze at load when no screen saver is defined

SVGHMI: update generated XSLT

SVGHMI: watchdog now calls OnStart and OnStop commands just after calling OnWatchdog. Therefore, OnWatchdog command should not restart browser by itself.

SVGHMI: server systematically resets session on connect, rather than waiting for client to ask for it

IDE: Fix DropTarget() methods that must return a bool with wxPython4.

IDE: Fix visibility of text in Debug panel when using dark theme.

IDE: Fix use of deprecated wx.NamedColour

Merged changes from default in wxPython4 branch

SVGHMI: update generated XSLT

SVGHMI: use messy test/projects/svghmi to give a try to ScreenSaver feature.

SVGHMI: cosmetic fix in Back widget

SVGHMI: add ScreenSaver feature. Automatically jumps to a page named "ScreenSaver" after timeout defined in page arguments.

Backed out changeset c6068b674b30

SVGHMI: update generated XSLT

SVGHMI: enable periodical reconnect when browser is QtWebengine, working around memory leak with websocket on Qt's embedded Chromium.

IDE: Added filtering in POU instance variables panel.

SVGHMI: add automatic reconnection of websocket with 1s reconnection delay after first attempt.

SVGHMI: prevent browser and watchdog commands to become zombie once finished.

SVGHMI: svghmi_references exemple: add missing PLC part to demonstrate assigning PLC/HMI variables when validating a modal dialog.

SVGHMI: cosmetic fixes

runtime: remove forgotten printf at starup when retain memory is restored.

OPC-UA client: add ApplicationUri parameter in case it differs from server's URI

Runtime: fix PLC not terminating when being Repaired (i.e purged).

IDE: fix exception when re-opening previously closed tab

IDE: fix SFC Action editor's text not visible on GTK3's dark themes.

IDE: fix action block editor's GridCellEditor. Consequence of wxPython4 bump.

Update README.md

removed undue blank lines

Update README.md

use back quote instead of simple quote for quoted strings.