QHexEditData: Fixed a bug when the data is retrieved from internal
buffer.
Initial work in order to make read/write methods thread safe.
read() and at() methods are simplified and they are faster now.

QHexEdit: Font changed from "Courier" to "Monospace".
indexOf() method should accept a qint64 value instead of bool
When Insert key is pressed, update the cursor immediately.
Highlight Ascii Character or Hex Byte when the cursor is changed.