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. Only the large icon is
replaced - smaller ones in the corners are not affected.

The decorator is enabled by default and can be disabled in
"Preferences/General/Appearance/Label Decorations/Scala Decorations"

This change only affects Scala files that are not on the classpath
because internally they are represented with a different object
compared to all other Scala files.

Fixes #1001975
Merge pull request #616 from sschaef/t1001975-enable-icon-for-files-not-on-classpath

Adds decorator extension point to change icons for Scala files