Adds decorator extension point to change icons for Scala files

Scala files which are not on the classpath of a project get displayed
with an icon that is made for Java files. This commit replaces the Java
icon with a different one by using a decorator.

The decorator is enabled by default and can be disabled in
"Preferences/General/Appearance/Label Decorations/Scala Decorations"

This change only affects Scala files not on the classpath because
internally they are represented with a different object compared to all
other Scala files.

Fixes #1001975