appDisplay: Consider drop targets on !currentPage as invalid

The iconGrid's getDropTarget() API supports dropping items to adjacent
pages just fine, but in the AppDisplay, we clip the grid and don't show
those adjacent pages. That doesn't stop getDropTarget() from picking
drop targets which are on adjacent pages though, so we need to filter
those out in the layer above.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2348>

js/ui/appDisplay.js

@@ -798,6 +798,7 @@ var BaseAppView = GObject.registerClass({
 
         // Dragging over invalid parts of the grid cancels the timeout
         if (item === source ||
+            page !== this._grid.currentPage ||
             dragLocation === IconGrid.DragLocation.INVALID ||
             dragLocation === IconGrid.DragLocation.ON_ICON) {
             this._removeDelayedMove();