Added a getLength function and the assert from #14

+ Added RichText::Line::getLength() which returns the line length as
  an std::size_t

+ Added an assert to RichText::Line::convertLinePosToLocal() to
  protect against out-of-bounds access to the Line (causing UB).
Renamed the index in the getLength function to text.
Added asserts to the get/setChar methods.