feat(rdp): text clipboard sharing in both directions

A CLIPRDR channel with a text-only backend (CF_UNICODETEXT): text copied
on the remote desktop is fetched and goes through the app's remote-
clipboard preference like VNC and OSC 52; local text is offered to the
remote when the desktop gains focus, on Ctrl+V and with Paste Clipboard
(which no longer types the text). Backend callbacks run inside the
active stage, so they queue requests that the session loop sends.

Verified against xrdp: the remote X clipboard read back the offered
text, and a remote copy arrived through on_clipboard.

rust/app/src/app.rs

@@ -528,7 +528,7 @@ impl App {
                 }
             }
             Event::DesktopFrame { .. } => {}
-            Event::VncClipboard { text, .. } => self.remote_clipboard(&text),
+            Event::DesktopClipboard { text, .. } => self.remote_clipboard(&text),
         }
     }