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/core/src/sessions/mod.rs

@@ -124,7 +124,7 @@ pub enum Event {
         session: String,
     },
     /// The remote desktop's clipboard changed.
-    VncClipboard {
+    DesktopClipboard {
         session: String,
         text: String,
     },