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