Userland: Accept drag_enter events for widgets supporting file drops

This patch will switch cursor to DragCopy when a user enters a widget
while dragging file(s), giving them a visual clue that it *might* be
dropped into this widget.

This is a rather naive approach, as the cursor icon will change for any
kind of file, as currently programs don't know the drag contents before
dropping it. But after all I think it's better than nothing. :^)

Userland/Applications/FontEditor/MainWidget.h

@@ -62,6 +62,7 @@ private:
     ErrorOr<void> create_undo_stack();
     ErrorOr<RefPtr<GUI::Window>> create_preview_window();
 
+    virtual void drag_enter_event(GUI::DragEvent&) override;
     virtual void drop_event(GUI::DropEvent&) override;
 
     void undo();