Add keyboard input

A wlr_keyboard_group is used to multiplex events from multiple
Rig up cursor events
Render using wlr_scene

sedna is pretty young, so it's a good candidate to adopt the new
wlr_scene API [1] without too many changes. For now only rendering is
offloaded to wlr_scene, because wlr_scene doesn't support anything else.
In the future, more features will be delegated (frame scheduling &
events handling, presentation-time, hardware planes management, etc).

[1]: https://github.com/swaywm/wlroots/pull/1966
Flesh out input riggings somewhat
Add -s to run a startup command
Upgrade to wlroots 0.9.0

Also perform a modeset if necessary
Delete subprojects/

The symlink'ed wlroots directory is local configuration.
Fix use-after-free on view destroy

The rendering loop tries to access the free'ed view.
Log Wayland display before starting it
Add and configure wlr_seat
Initial start on input device riggings
Fix xdg_toplevel_view_for_each_surface error
Rig up remainder of XDG shell view API
Rig up remainder of view abstraction API
Initialize logging before setting up server
Handle XDG toplevel destruction properly
Render surfaces
Rig up xdg view implementation
Add a few plug-and-play wlroots modules
Draft view interface, rig up xdg-shell somewhat