Fix problem with inserting menus in wxMDIParentFrame menu bar.

Account for the "Window" menu (and any other foreign windows lurking in our
menu bar) correctly in wxMenuBar::Insert() instead of the old code which
apparently tried to do something similar but didn't make much sense and didn't
work when trying to insert the menu at the position actually occupied by the
"Window" menu in the menu bar.

Closes #15662, #1732.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75243 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix replacing "Window" menu in wxMDIParentFrame in wxMSW.

When setting a new "Window" menu (as opposed to just modifying the existing
one which did work correctly), we shouldn't show it immediately in the menu
bar if there are no MDI children, this results in wrong UI and assert
failures.

Closes #15663.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75244 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775