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/event-editor/gcal-time-zone-dialog-row.blp

@@ -0,0 +1,10 @@
+using Gtk 4.0;
+using Adw 1;
+
+template $GcalTimeZoneDialogRow: Adw.ActionRow {
+  activatable: true;
+
+  styles [
+    "property",
+  ]
+}