views/month-cell: Add margins to overflow button

`margin-end` and `margin-bottom` have a 2px margin since the
`box-shadow` CSS property, which is used to mimic a border,
is offset by 1px.

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

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

@@ -60,6 +60,10 @@ template $GcalMonthCell: Adw.BreakpointBin {
       sensitive: false;
       clicked => $overflow_button_clicked_cb(template) not-swapped;
       tooltip-text: _("All Events");
+      margin-top: 1;
+      margin-start: 1;
+      margin-bottom: 2;
+      margin-end: 2;
 
       styles [
         "flat",