Set the width of the last status bar pane correctly in wxMSW.

Set the width of the last status bar pane correctly in wxMSW.

The total width of status bar panes must add up to the size of the status bar
as otherwise an extra unwanted border is drawn after the last pane and we did
have this border for status bar with a size grip.

So while we still use the width without the size grip for calculating the
fields widths, pass the width with the size grip to Windows to prevent this
from happening.

Also, don't pretend that the last field stretches up to the status bar edge
when it should end before the size grip and Windows even already helpfully
does it for us.

Closes #12655.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66236 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775