Semantic Highlighting now works on classfile source attachments

Created a `ScalaCompilationUnitEditor` that contains the logic common to
both the `ScalaSourceFileEditor` and the `ScalaClassFileEditor`. In particular,
the logic needed to install/dispose semantic highlighting is encapsulated in
`ScalaCompilationUnitEditor`.

It is worth pointing out that the initialization of the Semantic Highlighting
engine is different for the two editors. On the one hand, on an editable scala
source, semantic highlighting is triggered by the reconciler. On the other
hand, for a classfile source attachement, no reconciler is needed (because the
source file can't be edited), hence semantic highlighting needs to be forced at
initialization. This is why in `ScalaCompilationUnitEditor` we have the
following two additional abstract methods

* `forceSemanticHighlightingOnInstallment`
* `createSemantichHighlighter`

This commit comes with no tests because it is not possible to avoid the loading
of UI classes.

Fix #1000939
Integrated reviewers' feedback
Merge pull request #452 from dotta/issue/semantic-highlighting-on-attached-sources-1000939

Semantic Highlighting now works on source attachments