Interactiveprover

[florian-rabe]

This is what @SvenWille has done so far.

We're not going to merge it as is. But it's enough to start the discussion.
In particular, the files Solver.scala and SolverAlgorithms.scala are affected critically.

• Branchpoints and backtracking removed (was never used anyway)
• Stateful fields encapsulated in a class
• new functionality for reading out and setting the whole state (e.g., for undo/redo functionality)
• various hard-to-understand low-level changes that may or may not break things

@Jazzpirate Can you have a look?