Show confirmation dialog before doing a workspace wide clean

Fixes #1002043
Implementation of an indent guide + test suite

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
Fix indent guide indentation depth for tabs
Scala preference is chosen for indentation depth in Scala editors

Fixes #1000828
Merge pull request #634 from sschaef/t1002033-indent-guide

Implementation of an indent guide + test suite
Merge pull request #638 from sschaef/t1002043-disable-automatic-clean

Show confirmation dialog before doing a workspace wide clean