private BWindow::_StealMouseMessage() which makes sure only messages for the
preferred handler are stolen, and also, that nothing gets lost that shouldn't
get lost.
Also updates the fLastMouseMoved view in case the message is actually removed
due to GetMouse().
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@18601 a95241bf-73f2-0310-859d-f6bbb57e9c96