xdg-shell: add use case to handle change of app_id
LibreOffice is one big binary with explicit brandings for different application modules. This is represented by different WM_CLASS settings for a window on X11, based on the loaded document. As a result LibreOfiice already offers multiple desktop files with different icons and StartupWMClass entries.
This amendment of the set_app_id request just explicitly specifies this use case to change a windows app_id on runtime to change icon and grouping of a window as used by LibreOffice.