xdg-shell: use case to change the app ID at runtime

LibreOffice is one big binary with explicit brandings for different
application modules. This is represented in X11 by a different
WM_CLASS setting for a window. The WM_CLASS is changed based on the
loaded document at runtime. As a result LibreOffice already offers
multiple desktop files with different icons, StartupWMClass
entries and application names.

This amendment of the set_app_id request just explicitly specifies
the use case to change a surfaces' app ID at runtime, so a compositor
implementor is made aware of it. Just as the WM_CLASS, a change of
the app ID should result in an update of the propertes of a surface
depending on the app ID, like the window icon specified in the
desktop file or a re-grouping of the surfaces in a task manager.

Signed-off-by: Jan-Marek Glogowski <glogow@fbihome.de>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>