Debugger: SourceView cleanups.

SourceView:
- Now that we need to track the current text drawing position for syntax
  purposes anyways, get rid of the previous inefficient hack to handle
  tabstops, and instead adjust for them on the fly. This both avoids
  quite a bit of string copying/reformatting, and eliminates the need to
  adjust the syntax highlight offsets to compensate for the aforementioned
  reformatting.
- Fix syntax color for string literals.