[Tracker] A couple of issues in tracker-search-tool

[Luca Ferretti <elle uca libero it>]

1. Search button should be grayed out when search entry is empty.

        To reproduce: open Tracker Search Tool, the search entry is
        empty, but you can click on Search button.
        
        More info: I suppose the search button should also be grayed out
        after a search is performed, and clickable only when there is
        something new to search in text entry.
        
2. Clear the results list on text entry cleaning

        To reproduce: open Tracker Search Tool, enter text in search
        entry, search and find it, now clic the clear entry icon. The
        results list still show previous results.
        
        More info: waiting for a search-as-you-type feature, should we
        clear the results list after every text entry changes