Handle _NET_WM_STATE_STICKY, but only for floating containers. If this atom is set, the floating window will always be automatically moved to the currently active workspace of the output that it is on. This is the equivalent of a sticky note stuck to the monitor.

We will respect this atom upon managing a window as well as when we receive a request that changes the sticky state.

fixes #1455

testcases/t/116-nestedcons.t

@@ -47,6 +47,7 @@ my $ignore = \"";
 
 my $expected = {
     fullscreen_mode => 0,
+    sticky => $ignore,
     nodes => $ignore,
     window => undef,
     name => 'root',