Keep track of keyboard state during drag and drop

Keep track of keyboard state during drag and drop

* src/xterm.c (x_dnd_cleanup_drag_and_drop): Deselect for
keyboard state changes.
(x_dnd_begin_drag_and_drop): Select for keyboard state changes
when XKB is available.
(x_dnd_update_state, handle_one_xevent): Use current XKB state
if it is available.
(x_term_init): Reformat code a little.