Update mouse capture stack when showing modal dialog in wxGTK.

GTKReleaseMouseAndNotify(), which is called before showing a modal dialog in
wxGTK to ensure that the mouse does not remain captured by the window which
will be disabled by the modal dialog soon, should update the mouse capture
stack and so must call ReleaseMouse() to do it, not just DoReleaseMouse().

See #16647, #8657.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78076 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775