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"
Before this patch, it was impossible to differentiate between negotiations
resulting in client side decorations and no negotiations at all.
By adding an enum we are now able to differentiate between the two states.