Enable mouse tracking in wxQT, thanks @seandepagnier

This is needed so qt mouse events are triggered without a button pressed.
Note that I modified this to prevent a crash if m_qtWindow was null (reproducible with controls sample and the original patch from Sean)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77925 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix case where widgets may get hidden by qt, thanks @seandpagnier

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77926 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775