Enhanced Clock widget

* Use self.layout.width to determine if whole bar should be redrawn or not.
* More precise ticks algorithm (it ticks on the begin of the seconds, and
doesn't rely on gobject.timeout_add as it is recommended in its documentation,
see http://www.pygtk.org/docs/pygobject/glib-functions.html#function-glib--timeout-add
for detail)

Closes #185