OT: Stabilize operational transformation.

[scofalik]

The goal of this issue is to make a research on bugs and problems directly in operational transformation algorithms and also in related parts of engine.

After this issue is done, operational transformation in engine should be considered stable. This basically means that editor won't blow up on most set of deltas to transform, apply to model and render.

Still it is expected that there may be some more complicated/edgy scenarios that will result in an error or incorrect document state.