Indent multi line string literals based on Scala preferences

In the case that smart indentation is disabled, but the insertion of
tabs is enabled, a Java preference was considered.

Fixes #1002167
Disable tab to spaces converter of `TextViewer`

There exists a `TextViewer#setTabsToSpacesConverter` method, which sets
a converter each time the Java preference in "Java -> Code Style ->
Formatter -> Edit -> Indentation -> Tab policy" is changed. This does
not work together with the Scala IDE auto edits, which need to know if a
tab was pressed to handle it in the right way.
Merge pull request #730 from sschaef/t1002167

Disable tab to spaces converter in TextViewer