Sync with X server before simulating mouse clicks

Adding a call to XSync() to wxUIActionSimulatorX11Impl seems to fix the
problems with the focus snapping back to the control having it previously
after simulating a mouse click and changing the focus afterwards using
wxWindow::SetFocus(), as the uiaction sample did.

Admittedly, it's not really clear why does doing it help, but at the very
leasy this XSync() call shouldn't do any harm.

src/unix/uiactionx11.cpp

@@ -106,6 +106,10 @@ bool wxUIActionSimulatorX11Impl::SendButtonEvent(int button, bool isDown)
             return false;
     }
 
+    // Ensure that the event is received by the correct window by processing
+    // all pending events, notably mouse moves.
+    XSync(m_display, False /* don't discard */);
+
     return DoX11Button(xbutton, isDown);
 }