The ICCCM claims this is a "required" target for selection owners,
however several GTK clients (zenity, winecfg) don't seem to support it.
(So much for required.) Just manually grab all the supported targets
individually from the selection owner instead.
Fix for #50
Wine windows had WM_HINTS.input set to False, and expected use of the
WM_TAKE_FOCUS protocol, but Wayland input is much more absolute, so this
protocol is useless to us. Always focusing windows seems to be fine, so
just do that.
Should fix#35.
Sometimes, windows will have actually been destroyed before we even
processed the events for it, so if we try to do something like get
properties on windows (as we do whenever a window is mapped) we will get
an error from the X server, but this is fine - we can just ignore this.
Should fix#16, #20, #22, etc.
