Created DataModelController

DataModelController is responsible for keeping parsed subtitles integrity. It communicates every
change to the underlying data model via signals-slots mechanism. This way the place where data is
stored has been moved outside the View layer (GUI).
DataModelController returns copies of subtitles. This way each client may modify a local copy of
subtitles and request their update on when all changes are ready.
Fixed comment
SubConverter stores state of conversion no more.