[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/Wayland/wayland.c

@@ -108,9 +108,11 @@ static bool getCompositor(char * dst, size_t size)
 static bool waylandInit(const LG_DSInitParams params)
 {
   memset(&wlWm, 0, sizeof(wlWm));
+  memset(&wlCb, 0, sizeof(wlCb));
   LG_LOCK_INIT(wlWm.surfaceLock);
   LG_LOCK_INIT(wlWm.pendingHDRLock);
   LG_LOCK_INIT(wlWm.hdrLock);
+  LG_LOCK_INIT(wlCb.lock);
   wlWm.desktop        = WL_Desktops[0];
   atomic_init(&wlWm.lockActive, false);
   atomic_init(&wlWm.cmFeaturesDone, false);
@@ -207,6 +209,7 @@ static void waylandShutdown(void)
 
 static void waylandFree(void)
 {
+  waylandCBFree();
   waylandIdleFree();
   waylandPresentationFree();
   waylandWindowFree();
@@ -215,6 +218,7 @@ static void waylandFree(void)
   waylandColorMgmtFree();
   waylandRegistryFree();
   waylandCursorFree();
+  waylandPollFree();
   wl_display_disconnect(wlWm.display);
   LG_LOCK_FREE(wlWm.hdrLock);
   LG_LOCK_FREE(wlWm.pendingHDRLock);