Improve search capabilities for extensions

Currently the extension manager's search functionality is to reliant on the repository names to find extensions. For example a search for DeCA does not return the extension SlicerDenseCorrespondenceANalysis. Or search for mouse segmentation does not find SlicerMEMOs. More importantly, because repository names cannot contain spaces, long extension names looks weird. For example one naturally would like to type `Mesh to labelmap` not `MeshToLabelmap` to search.

We need better search capabilities that will search more of the fields...

@jcfr @lassoan

Start by tracing the extension manager's current repository-name search and reviewing the example queries in this issue: DeCA, mouse segmentation, and Mesh to labelmap. Define which additional extension fields should be searchable and consider the expected results for those examples; done means relevant extensions are found without relying on repository-name formatting.

Written by the indexing model from the issue text.