[StickyScrolling] Introduce enhancement point

[Christopher-Hermann]

In order to implement editor/language specific sticky lines provider, a new extension point is introduced.

See issues:

• Sticky scrolling resets on comment lines eclipse-platform/eclipse.platform.ui#2338
• [Sticky scrolling] Commented-out code is being incorrectly recognized eclipse-platform/eclipse.platform.ui#2128
• Sticky scrolling should allow to be fed by language-specific info/annotations eclipse-platform/eclipse.platform.ui#1950

The idea is that the package of the extension point is internal in the first place.

1. We from SAP will provide a extension for the SAP language ABAP.
2. In [Help Needed] Implement provider for sticky scrolling in JAVA/JDT eclipse-jdt/eclipse.jdt.ui#1851 we provide a implementation for JDT.

If both implementation works as expected, the API is most probably stable and we can change to package of the extension point to public usage.