Make AbstractMacroConverter public

AbstractMacroConverter is a usefull helper when you implement MacroConverter (allows 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

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.

When fixing this, please also fix those applications at the same time:

• https://github.com/xwikisas/application-confluence-migrator-pro/issues/124
• https://github.com/xwikisas/application-task/issues/106