Scala Installation Support Improvements

The commit messages are explicit.

Investigates (logs) for #1002243

Todo:
- [ ] DRY on ScalaInstallationAction (w.r.t DesiredScalaInstallationWidget, for example)
- [ ] API conformance check

pinging 
- @gkossakowski : this should let you auto-set a scala installation after an sbt eclipse run with withBundledScalaContainer:= false
- @mpociecha : this should let you multi-select scala projects and set their scala installation as a single action, i.e. Right-click/Scala/Set the Scala Installation

review by @dragos
I am not sure I understand the sequence of events. If you are setting the desired installation, why do you still create an error marker? Will the preference trigger a correct version of `-Xsource` and in turn another round of classpath checks?