a11y: calendar: add accessible name for next/prev month buttons

https://bugzilla.gnome.org/show_bug.cgi?id=706903
a11y: calendar: Month name should be navigable

https://bugzilla.gnome.org/show_bug.cgi?id=706903
a11y: calendar: full date string should be navigable

Also moved the set of label_actor of the menu some lines before,
to improve readability.

https://bugzilla.gnome.org/show_bug.cgi?id=706903