Introduced IArtifactFilter interface and corresponding artifactFilters
extension point that allows filtering of maven index entries. Updated
code-assist and artifact selection GUI to ignore/block filtered
entries.

Enabled decoration of maven index entries in Maven Repositories view
and Add Depenencies dialog, so filtered entries can appear visually
differently. Actual decoration is left up to artifact filter provider
bundle.

Signed-off-by: Igor Fedorenko <[email protected]>