Restore mouse capturing/releasing in wxPopupTransientWindow under MSW.

Restore mouse capturing/releasing in wxPopupTransientWindow under MSW.

Partially revert the changes of r69350 to restore EVT_IDLE handling of
wxPopupTransientWindow under MSW. It turns out that we must release the mouse
for the embedded controls to work.

It would be better to solve the problem differently, e.g. perhaps maybe only
releasing mouse capture when mouse is clicked as the current code is fragile
but for now at least restore the functionality that used to work.

Closes #13740.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69966 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775