Win: Focus issues when embedding into non-Qt apps

We embed Qt controls into native floating palettes of Adobe Illustrator. In certain situations (semi-expanded palettes, see the screenshot), the Qt controls don't get the focus when clicked. Namely, when WM_MOUSEACTIVATE is unhandled, then WM_SETFOCUS is never sent. Therefore the proposed patch adds handling of QtWindows::MouseActivateWindowEvent for embedded widgets:

        if (platformWindow->isEmbedded()) {
            *result = LRESULT(MA_ACTIVATE);
            return true;
{{        }}}