Fix crash on keyboard remove

If the compositor sent a wl_keyboard.leave and a new wl_seat.capabilities
without the keyboard bit we would end up deleting the Keyboard object
with a roundtrip in flight, ending up using freed memory later.
Destroy the callback when destroying the keyboard, and notify the
window system the active window is active no more.

Change-Id: Ie415ca62b46e0b8ef0fe4d09c8e7a8130928df90
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/plugins/platforms/wayland/qwaylandinputdevice.cpp

@@ -118,6 +118,10 @@ QWaylandInputDevice::Keyboard::~Keyboard()
 #ifndef QT_NO_WAYLAND_XKB
     releaseKeyMap();
 #endif
+    if (mFocus)
+        QWindowSystemInterface::handleWindowActivated(0);
+    if (mFocusCallback)
+        wl_callback_destroy(mFocusCallback);
     if (mParent->mVersion >= 3)
         wl_keyboard_release(object());
     else