Add wxLogFormatter to allow easier wxLog output customization.

Add wxLogFormatter to allow easier wxLog output customization.

Delegate the log string creation to wxLogFormatter. This allows defining a
custom object of a class derived from it to customize the log output instead
of having to override DoLogRecord() in wxLog itself.

Closes #13792.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70086 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775