GtkSpinner queues relayouts even when the stack is not displaying
it, and this causes excessive redrawing. This button design looks
obsolete and should eventually be reworked, but for now, let's
just stop spinning when the spinner is invisible.
Closes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/971
Mnemonics are useful for navigating through the interface more easily,
besides improving the accessibility. Since the calendar button is an
important button for using the application, it is interesting to have
a mnemonic associated with its label.
Using the letter C for this label seems reasoable, since there is no
conflict with the labels simulteneously active in the application, and
it is easy to remember.
Hari Rana