QHexEditData: Fixed a bug when reading data from the internal buffer.
Initial multithreading support for read/write methods.
Implemented device() method in order to get a QIODevice* rappresentation
of QHexEditData.
Some performance improvements when reading data.

QHexEdit: Changed font from "Courier" to "Monospace".
Update cursor immediately when Insert key is pressed.
Highlight Ascii Character or Hex Byte when cursor position is changed.
Some Code formatting.