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 code is never executed on my installation. The whole method is not called at all on project open. I *think* you need to tweak the call to `addResourceChangeListener` bitmask. My test case is pretty simple:

- sbt, set `withBundledClasspath := false`
- sbt eclipse
- import existing project