Refactor wxMSW: move some code from wxMenu to wxMenuItem.

This will allow reusing it for other wxMenuItem bitmap-related operations.

See #9388.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76190 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix setting menu item bitmaps after appending them in wxMSW.

Update the bitmap used by Windows when using non-owner-drawn items with
bitmaps.

Closes #9388.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76191 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775