[eclipse-pmc] API Addition: Triple-Click Support

[Daniel Megert <daniel_megert@xxxxxxxxxx>]

In order to provide triple click support for textual editors I need to add
several new APIs:
- ITripleClickStrategy with tripleClicked(ITextViewer)
- DefaultTripleClickStrategy
- ITextViewerExtension7 with addTripeClickStrategy()
- SourceViewerConfiguration.getTripleClickStrategy(...) returning a
DefaultTripleClickStrategy

GOALS/BENEFITS:
- single-line selection on triple click in all textual editors


CORRESPONDING BUG:
- [typing] Single line selection on triple-click
      https://bugs.eclipse.org/bugs/show_bug.cgi?id=14654


RISKS:
No known risks.


PERFORMANCE IMPACTS:
None.


Dani