Bug 553622 - Highlighting in context information no longer works

This change restores the previous behavior in case of a single
IContextInformationValidator, so it removes the regression.
The case of multiple validators will be treated in a separate change,
as it's not a regression but more a missing feature for mulitple completion
processors.

Change-Id: I9a826bc5ec43ea59c48d304b330d4a3ed2d7739b
Signed-off-by: Mickael Istria <mistria@redhat.com>