Draft: wayland: Add support for xdg-session-v1

This change uses slightly modified version of the upstream protocol proposal:

• clients are responsible for picking toplevel ids
• add_toplevel and restore_toplevel were merged in a single request to simplify the compositor side implementation and cut down the amount of corner cases, e.g. calling add_toplevel multiple times for the same toplevel, etc
• the surface arg has been removed in xdg_toplevel_session_v1.restored as it's redundant

some notes:

• it makes sense to move xdg_session_v1.remove_toplevel to xdg_toplevel_session_v1.remove in order to be consistent with xdg_session_manager_v1.get_session and xdg_session_v1.remove

Draft:

• use KSharedDataCache or something like that for session storage
• get the upstream protocol merged

https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/18

closes #113 (closed)