Improve drag atom computation

Improve drag atom computation

* src/xterm.c (xm_get_drag_window): Avoid leak if error occured
creating drag window.  Also use StructureNotifyMask instead of
ButtonPressMask.
(xm_get_drag_atom_1): Update.  Make EMACS_DRAG_ATOM a list of
atoms and use the first one that isn't currently owned.
(xm_get_drag_atom): Stop owning selection here.
(xm_setup_drag_info): Record chosen atom.
(x_dnd_cleanup_drag_and_drop, x_dnd_begin_drag_and_drop)
(x_dnd_update_state, handle_one_xevent, x_connection_closed):
Use chosen atom.
* src/xterm.h (struct x_display_info): New field
`motif_drag_atom_time'.