Preserve focus when window is minimized and restored in wxMSW.

Preserve focus when window is minimized and restored in wxMSW.

Add specific code to save and restore the focus when the window is minimized
and restored in wxMSW as the existing code in WM_ACTIVATE handler wasn't
enough because this event was generated too late when minimizing the window
(when it was already minimized and so the focus had been already lost) and too
early when restoring it (so the window was still minimized and restoring focus
failed).

This is still not perfect as we do in our code something Windows would be
expected to do automatically but for whatever reason, it doesn't do it for
wxWidgets programs, and this manual workaround at least prevents the annoying
total focus loss.

Closes #1599.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70513 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775