Extend `contentMergeViewers` extension point: add an attribute to
consider "linked" editor content type associations. The code checks if
contentMergeViewers contribution refers to such "linked" editor id in
extension, and check if editor supports other content types. In case
there are more content types supported by same "linked" editor, these
would be automatically considered as valid contentTypes for the
contentMergeViewers.

Updated CompareViewerSwitchingPane to consider more precise title
calculations for ContentMergeViewer instances (where concrete type info
is available after instantiation of a particular viewer).

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/1747