Clean-up non-editor code

Several cleanups, divided in different commits. Should fix #80.

Notably, a large chunk of code that should also be cleaned up is in preferences. However, it looks like most of it should be actually moved upstream in Scala IDE, as it is not general enough (for instance, a SyntaxColorPage is an object in sdt.core). Preferences were not addressed in this PR, as I think it makes more sense to do it in a dedicated PR.