WIP: Support for multiple virtual connectors

[misyltoad]

WIP, but should be getting there.

Still some to-dos on the side of cursor setting and misc. operations on the WaylandBackend.

[matte-schwartz]

couple issues I've had pop up while using this PR for you to keep track of (which you may already be aware of):

• gamescope --backend sdl -- vkcube a gamescope window never appears on Wayland desktop (tested w/ Plasma 6), and does not shut down with gamescopereaper without being manually terminated. --virtual-connector-strategy has no effect on the behavior. --backend wayland seems fine.
• in gamescope-session, when quitting games using the left side menu, the transition and display seem to hang on the shutting down game window screen until you toggle the side menus a few times. Guessing this is what you mean in d80cf55 ?

will edit this comment if I find anything else