The search text can now also match the contents of a file, besides
the file name.

This is done with the help of a Tracker query, using fts:match, which
matces both the contents of a file and the filename.

The user also has the option to choose whether to use or not the
Full Text Search. This can be done with a preference, which represents
the default option when opening a new tab/window or from the search
popover.

https://bugzilla.gnome.org/show_bug.cgi?id=775961