Document properties from UiLoader

[pawelsalawa]

In uiloader.cpp there are less known, undocumented features.

At the time of writing this, there is:

    registerPropertyHandler(new UiScriptingCombo());
    registerPropertyHandler(new UiScriptingEdit());
    registerPropertyHandler(new UiCustomIcon());
    registerPropertyHandler(new UiUrlButton());

These handlers allow declaring specific properties for widgets in the UI forms (for example in plugins) that are than handled by these handlers.

For example UiScriptingEdit adds "scriptingEdit" property, which enables syntax highlighting, the "Format SQL" menu entry and sets the configured editor font.