* Add possibility to override and exclude built-in keyboard layouts
* Add QT_VIRTUALKEYBOARD_LAYOUT_PATH environment variable specifying
 the layout location on the file system or inside a qrc resource
* Update documentation accordingly

[ChangeLog] Add support for external keyboard layouts, which allows overriding and exclusion of the built-in keyboard layouts

Task-number: QTBUG-54254
Change-Id: I960b942c031221d29dbdf1cabed78be8d32f43ef
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>