date-selector: always use 4 digits entry for years

Since GtkCalendar widget is a bit clunky to navigate fast between years,
using a four years digits it's better because we are still
very close to the change of the century.
month-view: fix draw and allocation on 6 rows months

Sometimes months expand 6 rows completely instead of the regular
5. March, 2015 is an example of it. Allocation of widgets, drawing
of indicators, placement of the overflow popover was wrong.
date-selector: fixed re-entry in ::set_date

This was causing double actualization of internal day,month,year
variables because GtkCalendar::day-selected signal handler is run by
default after the user connected ones.
manager: cleaner way of returning enabled sources