Enable syntax coloring for footer minings in unified diff

Line-header minings already applied syntax coloring via StyleRange
computation. Footer minings (used when the diff sits at the end of a
document without a trailing newline) were missing this: they fell back to
a plain super.draw() call for the text. They now use the same
computeStyleRanges path, transformFontStyleToFont for bold/italic, and
the same rendering loop as the header. A shared MouseClickConsumer
(via the new IUnifiedDiffCodeMining interface) also gives footer minings
the same double-click overlay behaviour the header already had.

Author: tobiasmelcher