Implementation of an indent guide + test suite

I found out that it is possible to draw arbitrary information on the editor area. This knowledge led to this PR, which is super awesome in my opinion. Screenshot:

![indent-guide](https://f.cloud.github.com/assets/488530/2061514/660717a8-8c57-11e3-8093-23d0b36ad60e.png)

Maybe I'm the only guy who needs that feature, but if I am, don't tell me, otherwise I'm disappointed. ;)

---

The indent guide is enabled by default and can be disabled in
  Preferences -> Scala -> Editor

The implementation tries to redraw as less as possible to guarantee no
delay while typing - some short tests measured a drawing time of 1ms
while typing and 20-30ms when a new file is opened.

Furthermore, the editor preference page is redesigned.

Fixes #1002033