Right now you can only enter some terms in the entry, and trackerd search these terms applying the "And" logical operator.

I'm going to attach a patch that allows entering search terms like "foo and bar or baz". It's implemented by an expression tree, which will construct a query tree by parsing the string with search term and "and/or" logical operators (with the "And" operator taking precedence on the "Or" one). This query tree will be used later to compose the results in a single search hits list.

In the patch, this is implemented in tracker-query-tree.[ch], which replaces all the SearchQuery logic.

The only think missing, but I think should be safe to do, is to mark these logical operators as translatable, so people can search naturally in their own language