TI-Basic debugging enhancements

[adriweb]

Follow-up on #286

• Breakpoints (on source lines)
• debugBasicStepNext needs to be finished
• Code viewer
  • new code navigation features (goto/label jump) => probably to include in the normal/variable viewer as well
• Adding annotations/comments to lines (non-persistent?)
• Variable viewer
  • live refresh
  • displaying old and new value when modified
  • watchpoints if we can find a way to do that - break on change, at least
  • custom aliases/labels to var names

I'm not sure if the code viewer part should also be able to edit things (later?)... I guess not, at least not until #116 is done somehow.