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
  • [ ] 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.