Improve reviewing order and speed

[Humorloos]

Depends on:

• cherry-pick xmind api and sqlite backend from #5 #34

I am currently working an a new algorithm that will internally represent the concept map as a graph in anki to make use of graph specific algorithms for a new and enhanced review order. This will fix long loading times for answers with many crosslinks, hopefully improve the reviewing order especially when crosslinks are concerned and allow for further more sophisticated updates in the future (e.g. bidirectional questions)

[Humorloos]

The current implementation is incomplete and has some major problems, so this issue now depends on #34 before it can be implemented