search-engine-tracker: Fix the content search in 'shell overview'

Fix a bug that is not possible to search by content in the shell overview.

In the Nautilus interface, it is possible to search both by filenames and by content. But in shell overview not.

This bug was introduced in commit f4942473, but this new commit doesn't not restores the previous behavior in commit f4942473.

Fixes: #1437 (closed)