xwayland: Improve desktop integration (all-in-one)

This is the different merge requests !895 (closed) + !896 (closed) + !897 (closed) grouped all together in one merge request.

Managing 3 different but related merged requests was not practical, so I figured I'd rather put all them in a single MR instead.

This adds:

• a new command line option -geometry to set the size when running rootful
• a new command line option -host-grab to disable the keyboard shortcuts and confine the pointer on the host so that Xwayland can receive all keyboard events.
• CTRL+SHIFT to grab or release the keyboard and pointer, similar to Xephyr
• a new command line option -fullscreen to run Xwayland rootful fullscreen. Coupled with -geometry and -host-grab, this allows to run a full X11 desktop in Xwayland on a dedicated monitor with XRandr modes emulated (including resolutions higher than the actual output resolution)
• a meaningful title for the xdg_surface, it's nicer when running rootful.
• an xdg-toplevel listener so that the compositor can close the Xwayland rootful window
• the app_id and installs desktop file to run Xwayland
• optional libdecor support (with an -decorate option) for decorating the Xwayland rootful window

Closes:

• #1338 (closed)
• #1332 (closed)