Spawn a thread to fake do our XDND polling

We cannot integrate this into our event loop like we planned, because
Wine a) grabs the mouse pointer so we cannot do that, and b) blocks the
GUI thread. So instead we will spawn our own thread and do polling based
XDND. When Wine's tracker window gets destroyed, we know that the left
mouse button has been released.

src/wine-host/editor.h

@@ -33,6 +33,12 @@
 #include "utils.h"
 #include "xdnd-proxy.h"
 
+/**
+ * The most significant bit in an X11 event's response type is used to indicate
+ * the event source.
+ */
+constexpr uint8_t xcb_event_type_mask = 0b0111'1111;
+
 /**
  * The maximum number of Win32 messages to handle per message loop. This is
  * needed because otherwise some plugins can run into an infinite loop. I've