views/month-cell: Set accessibility properties for overflow button

This uses the "toggle-button" role instead of GtkToggleButton widget,
as GtkToggleButton remains toggled after closing the month popover.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/598>

src/gui/views/gcal-month-cell.blp

@@ -56,6 +56,7 @@ template $GcalMonthCell: Adw.BreakpointBin {
 
     [overlay]
     Button overflow_button {
+      accessible-role: toggle_button;
       valign: end;
       sensitive: false;
       clicked => $overflow_button_clicked_cb(template) not-swapped;
@@ -65,6 +66,11 @@ template $GcalMonthCell: Adw.BreakpointBin {
       margin-bottom: 2;
       margin-end: 2;
 
+      accessibility {
+        has-popup: true;
+        labelled-by: overflow_inscription;
+      }
+
       styles [
         "flat",
         "overflow",