Create Single- and Multi-line Scroll Controllers

As far as widgets should know for drawing, what region is visible at the certain moment, this "knowledge" is supposed to be implemented with ScrollController. There is a simple ScrollController, which can do this for a single line. This controller will be renamed to SingleLineScrollController, and another one MultiLineScrollController will be created. 

MultiLineScrollController contains a list of SingleLineScrollController_s _(QList<SingleLineScrollController*>). Each controller from this list knows visible range for one wrapped line only.