use libxkbcommon-x11 instead of libX11

Conflicts:
	i3lock.c
little cleanups thanks to Ran Benita’s review
listen for XKB events instead of the MappingNotify event (Thanks Ran)
remove obsolete xkb_state_update_key() calls (Thanks Ran)
update the changelog/readme for 2.6
Die when the X11 connection breaks during runtime (Thanks Eduan)
Ignore password validation is pam is in wrong state

This add a protection against wrong password spamming