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

src/window-rules.c

@@ -128,6 +128,10 @@ window_rules_get_property(struct view *view, const char *property)
 					&& !strcasecmp(property, "ignoreFocusRequest")) {
 				return rule->ignore_focus_request;
 			}
+			if (rule->fixed_position
+					&& !strcasecmp(property, "fixedPosition")) {
+				return rule->fixed_position;
+			}
 		}
 	}
 	return LAB_PROP_UNSPECIFIED;