Support for compare and merge viewers which show the content side-by-side.

## Package Specification

The ContentMergeViewer is an abstract compare and merge viewer with two side-by-side content areas and an optional content area for a common ancestor (for three-way compare). Because the implementation makes no assumptions about the content type it is a subclass responsibility to deal with a specific type. Its subclass ImageMergeViewer in package org.eclipse.compare.internal shows how to base a simple mergeviewer for images on ContentMergeViewer.

A ContentMergeViewer accesses its model by means of a content provider which must implement the IMergeViewerContentProvider interface.

The TextMergeViewer is the standard concrete subclass of ContentMergeViewer for comparing and merging text content.
A text merge viewer uses the org.eclipse.compare.rangedifferencer.RangeDifferencer to perform a textual, line-by-line comparison of two (or three) input documents. For text lines that differ the TextMergeViewer uses an ITokenComparator to find longest sequences of matching and non-matching tokens. The TextMergeViewer's default token compare works on characters separated by whitespace. If a different strategy is needed (for example, Java tokens in a Java-aware merge viewer), clients can create their own token comparators by implementing the ITokenComparator interface.

The TextMergeViewer not only works on whole documents but on subranges of documents too. In this case the viewer's input must be an IDocumentRange instead of an IDocument.