handle DefaultCallback() like addCallback()

[Rainer-Lang]

WIP - DO NOT MERGE

[Rainer-Lang]

WIP - we have to do here much more
not working: loadService.showCallback(loadingCallback);
TODO: don't use Class better use Callback - workaround: We could use getClass()

[KingJA]

@Rainer-Lang Why you want do like this loadService.showCallback(loadingCallback); , if you config LoadSir in Application, then how to get the object of Callback when you want to call showCallback(obj) in Activity of Fragmetn? new Callback? Could you tell me the reason.

[Rainer-Lang]

I answered you in Telegram. I think there could be something to discuss more in detail.

BTW: This PR shouldn't be merged.