Implement cross overview drag & drop

The gnome-panel allows the user to hover over a tasklist entry
while draging to activate a minimized or obscured window and drop onto it.

Implement a similar behaviour by allowing draging to the activities button or
the hotcorner (and thus opening the overview), which allows the user to
activate any window (even on different workspaces) as a drop target.

https://bugzilla.gnome.org/show_bug.cgi?id=601731

src/shell-global-private.h

@@ -11,4 +11,6 @@ void _shell_global_set_plugin      (ShellGlobal  *global,
 void _shell_global_set_gjs_context (ShellGlobal  *global,
                                     GjsContext   *context);
 
+gboolean _shell_global_check_xdnd_event (ShellGlobal  *global,
+                                         XEvent       *xev);
 #endif /* __SHELL_GLOBAL_PRIVATE_H__ */