Make AbstractMacroConverter and ConfluenceConverter public

AbstractMacroConverter is a useful helper when you implement MacroConverter (allows third parties to alter the way macros are converted by confluence-xml), and ConfluenceConverter can be difficult to avoid. Both are internal and should be made public.

Pandora's box is open anyway, they are used by at least this third party project:

• https://github.com/xwikisas/application-confluence-migrator-pro

This internal APIs have moved a bit in recent times, but it's mostly stable and it's fine to expose it officially. And the use case is already there.

Confluence Migrator Pro's related issue:

• https://github.com/xwikisas/application-confluence-migrator-pro/issues/124