Scala Installation Support Improvements

The commit messages are explicit.

Investigates (logs) for #1002243

~~Todo:~~
- ~~[x] DRY on ScalaInstallationAction (w.r.t DesiredScalaInstallationWidget, for example)~~
- ~~[x] 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've tested that test case and consistently hit that method on the Play framework (by default a 2.10.4 project seen from sbt). What do you use for testing ?