xbase: use selected_item() from the base class

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: add set_enabled() helper

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: make "Spacebar" toggle the enabled checkbox

Signed-off-by: David Aguilar <davvid@gmail.com>