[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/desktops/xdg/xdg.c

@@ -300,7 +300,7 @@ void xdg_pollWait(struct wl_display * display, int epollFd,
   bool sawDisplay = false;
   for (int i = 0; i < count; ++i) {
     struct WaylandPoll * poll = events[i].data.ptr;
-    if (!poll->removed)
+    if (!atomic_load_explicit(&poll->removed, memory_order_acquire))
       poll->callback(events[i].events, poll->opaque);
     if (poll->fd == state.displayFd)
       sawDisplay = true;