window-rules: add fixedPosition property

...to address regression introduced by 57075ce and enables panel/desktop
clients which rely on window rules to remain in the same position when
the usable-area changes (normally because an exclusive layer-shell
clients is started/finished).

Also disallows interactive move/resize, for example by alt +
mouse-press.

Fixes: #1235

include/window-rules.h

@@ -30,6 +30,7 @@ struct window_rule {
 	enum property skip_taskbar;
 	enum property skip_window_switcher;
 	enum property ignore_focus_request;
+	enum property fixed_position;
 
 	struct wl_list link; /* struct rcxml.window_rules */
 };