ext_toplevel_state: new protocol implementation

[dkondor]

Based on the wlroots draft implementation of the protocol by @tokyo4j (available here), backported to wlroots 0.19: https://github.com/dkondor/wlroots/tree/ext_toplevel

Note: this is currently a bit incomplete (output information is not sent and additional states like always on top are not supported); I'll update it as I have some time. Also, currently, the -state protocol is always enabled -- it could make sense to add a setting for it in case a user would like to only expose the -info protocol.

I've tested so far only briefly with my Cairo-Dock implementation of the same protocol: https://github.com/Cairo-Dock/cairo-dock-core/pull/192 and will test more in the coming weeks.