#4 Enable metadata-based feature-list filtering in update UI

[vowitaf]

As the number of plugins/features in the IVCSF grows, people will need a
better way to figure out what plugins/features they would like to
download. For this, we need the ability to
- attach arbitrary metadata to plugins/features (using extension points).
- perform filtering of viewers that display plugins/features based on such
metadata.
Currently Eclipse has filtering built into its preferences UI but it is internal.
It may nor may not be made API (see: https://bugs.eclipse.org/bugs/
show_bug.cgi?id=74795). In any case, making the filter itself is not hard
at all. Embedding it into the update UI seems to be impossible at the
moment. Is there a solution?