Draft: clients/stacking: Add xdg-session-management-v1

This MR contains an work in progress example client implementation of xdg-session-management-v1 (wayland-protocols!18) with a tiny modification (wayland-protocols!18 (comment 2112792)).

It allows you to create sessions, add toplevels to sessions, then restore toplevels once they've been closed.

Further work:

• Split client out of stacking into a new example
• Ability to test the restoration of session records rather than just toplevels
• Ability to test toplevel record removal
• Testing and cleanup
• I'm working on a compositor implementation

Issues I don't intend to solve:

• Storing session information between runs