Move screencasting into a separate service process

Move the screencasting into a separate D-Bus service process, using
PipeWire instead of Clutter API. The service is implemented in
Javascript using the dbusService.js helper, and implements the same API
as was done by screencast.js and the corresponding C code.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1372

js/dbusServices/screencast/main.js

@@ -0,0 +1,11 @@
+/* exported main */
+
+const { DBusService } = imports.dbusService;
+const { ScreencastService } = imports.screencastService;
+
+function main() {
+    const service = new DBusService(
+        'org.gnome.Shell.Screencast',
+        new ScreencastService());
+    service.run();
+}