common/date-chooser: Jump to first day of month on button press

Encode the day in the combined choice value and use the callbacks
introduced in the previous commit to always jump to the first day
of the month when the next/previous month buttons are clicked.

Closes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1103
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/369>

src/gui/common/gcal-date-chooser.blp

@@ -41,8 +41,10 @@ template $GcalDateChooser: Adw.Bin {
     $GcalMultiChoice combined_choice {
       visible: bind $get_combined_choice_visible(template.show-heading, template.split-month-year) as <bool>;
       halign: fill;
-      min-value: 12;
-      max-value: 119999;
+      // From January 1st, 1 (32 * (1 * 12 + 0) + 0):
+      min-value: 384;
+      // …to December 31st, 9999 (32 * (9999×12+11) + 30):
+      max-value: 3839998;
       popover: popover;
       animate: false;
       value: 0;