Implement EVT_MOUSE_CAPTURE_LOST handling for wxPopupTransientWindow.

Implement EVT_MOUSE_CAPTURE_LOST handling for wxPopupTransientWindow.

Handle mouse capture lost events instead of polling for the mouse status in
EVT_IDLE handler. This is not only more efficient but also catches the cases
when the capture was lost before OnIdle() could be executed which could result
in assertion failures and, before the previous commit, even crashes.

The idle-time code is still used for wxOSX/Carbon because it doesn't seem to
generate mouse capture loss events currently -- but should be removed as soon
as support for these events is added.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69350 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775