Use C locale for numbers in wx(File)Config.

Using the current locale decimal point in config files results in problems
when moving the files to another machine or even using a different locale on
the same one, so don't do it.

Always write the numbers using C locale and try to read them in C locale too
first, but also try the current locale if we failed for backwards
compatibility and to be tolerant with users who edit their config files by
hand.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64450 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -450,6 +450,7 @@ All:
   resources.
 - Added wxMessageQueue::Clear().
 - Added wxConfig::Read(float *) overload (Terry Farnham).
+- Always use decimal point (and not the current locale separator) in wxConfig.
 
 Unix: