PLCObject : reorganize code in StartPLC(), add a PreStartPLC() empty method, to be overloaded by runtime extensions.

Solve import order problem when POULibrary imports ConfigTreeNode because of UserAddressedException. Now UserAddressedException is declared in POULibrary.py, as a quick and dirty fix, but some independant module should be created for beremiz exceptions in general.

SVGHMI: stop using eval in change_hmi_value, apparently slowly leaking memory.

SVGHMI: Switch widget was picking sub-elements from original SVG, not the result SVG ehwe clones are unlinked, thus making problems with svg:use based widgets.

SVGHMI: When unlinking a clone of a group, do not create additional group, but merge transformation and styles, and replace label. This allows svg:use based widgets to directly access elements of svg:use targeted element.

SVGHMI: Applies clone unlinking to widget themselves, not only to clones inside widgets.

merge

SVGHMI: Makes error when HMI tree is not well formed. Prevents multiple and non-first HMI_NODE, and ensure that all paths in HMI tree are unique.

Added FatalError call to Libraries code gen, so that they can emit messages addressed like config tree nodes do.

SVGHMI: more Widget properties declared ES6 style.

SVGHMI: Switch widget now use class declaration

SVGHMI: Update Button widget to class declaration, and add a button widget in tests/svghmi.

SVGHMI: explicit handling of undefined maximum widget update frequency

SVGHMI: removed dead code

SVGHMI: Back button updated to class style