Fix #25 - Caret is kept at the same position after evaluation

[dotta]

• Empty lines are no longer stripped, the reasons are:
  • Maintains correct indentation of empty lines.
  • Correct handling of caret position (i.e., if the caret is in the middle of an empty line, it should stay there after evaluation).
• The new caret position can never be after the old caret position (this should motivate the change in Mixer.scala)

(let's test the PR validator)