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
This has no effect when trying to switch from 2.11 to 2.10 (it works from the preference window). I think it's because the project settings isn't correctly set.

the `USE_PROJECT_SETTINGS_PREFERENCE` should pe set to `projectSpecificStorag`, not the plain storage (that can forward to one or the other, depending on exactly this setting). Right now I don't see a `projectSpecificStorage` in the API, so might need to use the internal `ScalaProject` type, or add this to the API.