Update GcalMultiChoice label when format callback changes

Currently the label text is only updated when set_value is called and
the new value is different to the current value. So if a format callback
is configured after the GcalMultiChoice is created, this won't be
applied until the value is changed to something other than the default.

The problem in issue #100 is that the default value for "month" is
0=>January. gcal_date_chooser_init sets the format callback and then
calls gcal_multi_choice_set_value with value==0 which won't update the
label.

Fix by having gcal_multi_choice_set_format_callback update the label
text with the result of applying the format callback to the current
value.

Fixes #100
3 jobs for issue100 in 6 minutes and 54 seconds (queued for 2 seconds)