MacroConverter is a useful interface allowing third parties to alter the way macros are converted by confluence-xml. But it's internal. Let's make it public?

Pandora's box is open anyway, MacroConverter and AbstractMacroConverter are used by these two third party projects:

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

Those internal APIs have moved a bit in recent times but I think they are mostly stable and it's fine to expose them officially. And the use case is already there.