feat(xdp): add portal-based ValentClipboardAdapter

Add a clipboard adapter based on `org.freedesktop.portal.Clipboard`
to eventually replace the Mutter plugin.

closes #624

Author: andyholmes

src/plugins/xdp/meson.build

@@ -12,6 +12,7 @@ plugin_xdp_deps = [
 plugin_xdp_sources = files([
   'xdp-plugin.c',
   'valent-xdp-background.c',
+  'valent-xdp-clipboard.c',
   'valent-xdp-input.c',
   'valent-xdp-session.c',
   'valent-xdp-utils.c',