workspace: Invalidate the current layout when windows are added or removed

If windows are removed or added, we shouldn't keep the old layout, as it's
not valid anymore. If windows are removed, this is especially bad, as the
rows contain references to the removed window objects, causing crashes.

https://bugzilla.gnome.org/show_bug.cgi?id=698622

js/ui/workspace.js

@@ -1257,6 +1257,7 @@ const Workspace = new Lang.Class({
         this._cursorX = x;
         this._cursorY = y;
 
+        this._currentLayout = null;
         this._repositionWindowsId = Mainloop.timeout_add(750,
             Lang.bind(this, this._delayedWindowRepositioning));
     },
@@ -1304,6 +1305,7 @@ const Workspace = new Lang.Class({
             clone.overlay.relayout(false);
         }
 
+        this._currentLayout = null;
         this._recalculateWindowPositions(WindowPositionFlags.ANIMATE);
     },