Delta transformation: move delta normalization to delta classes

[scofalik]

(Follow-up to ckeditor/ckeditor5-engine#977)

In ckeditor/ckeditor5-engine#977 I introduced "deltas normalization". To keep things simple and speed up work on the PR, I put it in one big function getNormalizedDeltas that takes DeltaClass (class object) and operations array and basing on this, the function generates proper deltas. Inside the function there is a big switch that checks what is the DeltaClass.

This solution has one big weakness -- it is impossible to extend it from outside. Of course the other weakness being that it is natural that this code be placed in static methods of delta classes (especially since the function is just a big switch block).

Hence, I propose to split this function and move the code to the deltas.

[scofalik]

Since we will change deltas to operations, this issue will be invalid.