cleanup: Use non-deprecated key symbols

Clutter originally cluttered its namespace with key symbols, before
prefixing all symbols with KEY. We still use the unprefixed symbols
occasionally, replace them so mutter can drop the deprecated symbols.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/808

js/ui/dnd.js

@@ -267,7 +267,7 @@ var _Draggable = class _Draggable {
         // dragging and ignore all other key presses.
         } else if (event.type() == Clutter.EventType.KEY_PRESS && this._dragState == DragState.DRAGGING) {
             let symbol = event.get_key_symbol();
-            if (symbol == Clutter.Escape) {
+            if (symbol == Clutter.KEY_Escape) {
                 this._cancelDrag(event.get_time());
                 return Clutter.EVENT_STOP;
             }