Add support for "%V" and "%G" to wxDateTime::Format().

This is useful for creating ISO 8601 week number based stamps.

Closes #11857.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76958 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix static initialization order problem in wxStopWatch under MSW.

Don't rely on the static global gs_perfCounter being already initialized when
wxStopWatch::DoStart() is called, this may not be the case if wxStopWatch
variable is global.

Work around this by wrapping the variable inside a function to ensure that it
is initialized before being used.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76959 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775