Fix GTK (2.22+ and 3.0+) compatibility in FmMenuToolItem widget.
Menu item editing appeared to be broken now, fixing it.

The GOutputStream for VFS menu:// was severely broken after changes
on menu XML manipulations, and leaded to failure. Commit fixes it.
Eliminate GTK3 warning about gtk_widget_get_pointer() deprecation.

Use gdk_window_get_device_position() instead.
Remove unused GtkRequisition from fm-menu-tool-item.c menu positioning func.