Bug 703272 - list of windows in Windows menu outdated

Only set the menu once per window!
When combined with fixing the autorelease pool problem of Byg 698334,
the window menu now behaves correctly.

BP

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23098 57a11ea4-9604-0410-9ed3-97b8803252fd