calendar: Don't ever force reload

https://bugzilla.gnome.org/show_bug.cgi?id=720298
calendar: Don't rebuild the entire calendar widget when choosing a date

It's inefficient and wasteful. Combined with the JS GC not being great,
it leaks around 100 actors waiting to be GC'd. Yikes.

https://bugzilla.gnome.org/show_bug.cgi?id=720298