project wide: clean up event listeners on shutdown

project wide: clean up event listeners on shutdown

This ensures all event listeners are removed before the emitting
wlroots object is being destroyed. This will be enforced with asserts
in wlroots 0.19 but there is no reason to not do it right now either.

This change in wlroots 0.19 is implemented via commit
8f56f7ca43257cc05c7c4eb57a0f541e05cf9a79
"Assert (almost all) signals have no attached listeners on destroy"