1. New Calendar Widget

The current GtkCalendar has some flaws:

One of these for example, is that all over the API the month value is assumed to be in the 0-11 range. The same happens for the "month" property, which prevents us from just adding a new API using the Glib types.

Currently, 0 is used for "january" which is just not logical. Days are in the 1-31 range, which is good.

Moreover, the current GtkCalendar doesn't map the Glib types for dates: