Support external XAML files with keyboard layout


XAML files are powerful enough to fully provide keyboard layouts, including active ("sticky") keys highlighting (using WPF databinding). Need to be able to load them during runtime. This may, but does not have to, require implementing VirtualKeyboard keypresses as WPF Commands (instead of/in addition to click handler).
The goal is maximum flexibility. User must be easily able to configure keyboard layout file. Later we will think about configuration options and selecting/switching layouts.