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
Oh, yes, you're right ! there's a catch : the storage writes to project-specific storage when created from a ScopedPreferenceStore, but for that to be the case, the project needs to already be using project-specific settings (which was unfortunately already the case in my tests - sorry -, but isn't true in general)