[WIP] wlr_primary_selection: Allow multiple offers per source

Create as many offers as there are selection devices, so that we do not send a client multiple times the same "new_id" offer (which would make it crash)

Note something is weird with this and X11; once I've selected something in firefox then pastes to X11 do not work anymore (even from other wayland apps); I don't understand what though so could use some help with that.

Part of #1041 (closed) (this is not complete as it only does primary_selection, data_source needs a similar fix... but data source uses source->offer everywhere (like in drag n drop as well) so such an approach is going to be a royal pain)

Cc @aereaux as he said he would work on this a couple of weeks ago, if you want to take over please do (I can find something else to do :P); or if you just have thought about it and have an opinion it's welcome as well :)