[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/include/app.h

@@ -165,9 +165,11 @@ void app_overlayConfigRegisterTab(const char * title,
 
 void app_clipboardRelease(void);
 void app_clipboardNotifyTypes(const LG_ClipboardData types[], int count);
-void app_clipboardNotifySize(const LG_ClipboardData type, size_t size);
-void app_clipboardData(const LG_ClipboardData type, uint8_t * data, size_t size);
-bool app_clipboardRequest(const LG_ClipboardReplyFn replyFn, void * opaque);
+void app_clipboardData(LG_ClipboardRequest request,
+    LG_ClipboardData type, const void * data, size_t size);
+void app_clipboardAbort(LG_ClipboardRequest request);
+bool app_clipboardRequest(LG_ClipboardData type,
+    LG_ClipboardReplyFn replyFn, void * opaque);
 
 /**
  * Show an alert on screen