Improve handling of input methods during drag-and-drop

Improve handling of input methods during drag-and-drop

* src/xterm.c (x_dnd_cleanup_drag_and_drop)
(x_dnd_begin_drag_and_drop): Stop removing IC and filter events
before dispatching them.
(x_filter_event): Actually filter events during DND.
(XTread_socket): Verify GenericEvents are actually input
extension events before ignoring them.