Follow up of https://github.com/enkore/i3pystatus/pull/149#issuecomment-67712619 .

This commit removes and replaces all the old methods 'on_*' by settings
with the same name. The old methods were renamed into more explicit names that can be used for the callbacks like "next_song","mute" etc...
For instance, you can test with:
status.register("clock",
	format=[
	    ("Format 0",'Europe/London'),
	    ("%a %-d Format 1",'Europe/Dublin'),
	    "%a %-d %b %X format 2",
	    ("%a %-d %b %X format 3", 'Europe/Paris'),
	],
	on_leftclick= ["urxvtc"] , # launch urxvtc on left click
	on_rightclick= ["scroll_format", 2] , # update format by steps of 2
	log_level=logging.DEBUG,
	)

This way much code could be removed from other modules, though I did it only for the clock module here.