[client] clipboard: add transport provider interface

Route clipboard exchange through a provider interface. Connected
transports take priority while SPICE remains the fallback.

Track asynchronous X11 and Wayland transfers by request, preserving
clipboard ownership across provider changes.

Use PureSpice clipboard status notifications for capability and
lifecycle updates.

client/displayservers/X11/clipboard.h

@@ -31,6 +31,6 @@ bool x11CBEventThread(const XEvent * xe);
 bool x11CBInit(void);
 void x11CBNotice(LG_ClipboardData type);
 void x11CBRelease(void);
-void x11CBRequest(LG_ClipboardData type);
+void x11CBRequest(LG_ClipboardRequest request, LG_ClipboardData type);
 
 #endif