event-editor/multi-choice: Make up/down buttons unfocusable

Spin buttons' increment/decrement buttons are standardly not made
to be accessible/focusable.

This copies the behavior from the GtkSpinButton widget.

Related: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1036
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/603>

src/gui/event-editor/gcal-multi-choice.blp

@@ -7,6 +7,7 @@ template $GcalMultiChoice: Box {
   Button down_button {
     icon-name: "pan-start-symbolic";
     clicked => $button_clicked_cb();
+    can-focus: false;
 
     styles [
       "back-button",
@@ -46,6 +47,7 @@ template $GcalMultiChoice: Box {
   Button up_button {
     icon-name: "pan-end-symbolic";
     clicked => $button_clicked_cb();
+    can-focus: false;
 
     styles [
       "circular",