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/importer/gcal-import-file-row.blp

@@ -0,0 +1,12 @@
+using Gtk 4.0;
+using Adw 1;
+
+template $GcalImportFileRow: Adw.Bin {
+  ListBox events_listbox {
+    selection-mode: none;
+
+    styles [
+      "boxed-list",
+    ]
+  }
+}