Add search look ahead support

[travolin]

Add the ability for typed look ahead. This can be done in two forms term look ahead and phrase look ahead. In the case of term look ahead, if the term being typed is a substring of a known term in the index the GUI should provide a suggestion based on the most prevalent term that matches. In the case of phrases, the same full word matching can be used for phrases found in documents.

This feature should be able to be turned on and off via the user settings.