gui: Port to Blueprint

This was mostly done with `blueprint-compiler port`.

Since Blueprint doesn't know about custom templates, I had to manually change some values from strings to literals.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/567>

src/gui/common/gcal-calendar-combo-row.blp

@@ -0,0 +1,13 @@
+using Gtk 4.0;
+using Adw 1;
+
+template $GcalCalendarComboRow: Adw.ComboRow {
+  title: _("Calendar");
+  title-lines: 1;
+
+  factory: SignalListItemFactory {
+    setup => $calendar_item_setup_cb();
+    bind => $calendar_item_bind_cb();
+    unbind => $calendar_item_unbind_cb();
+  };
+}