Don't pop explicitly changed status messages.

Don't pop explicitly changed status messages.

Calls to SetStatusText() in between Push/PopStatusText() were simply lost
before, now the text explicitly changed by SetStatusText() is preserved by the
next call to PopStatusText().

This required adding a new virtual method, called DoUpdateStatusText(), which
is now implemented in all the derived classes instead of overriding
SetStatusText() (on the bright side, it doesn't need to do any checks already
done in the base class any more).

Also fix PushStatusText() to actually show the text being pushed at all under
wxMSW as a side effect.

And further reduce code duplication between wxStatusBarBase and the derived
classes.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61624 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775