Fixes search field (Issue #65)

Cmd+F now marks the text of the search field if the filterbar is already visible and sets the focus. So the user can now start typing it's new search term immediately after pressing Cmd+F instead of clearing the field manually.

Also the hitting the Return key while entering the search term now fires the updateFilter. Previously the filter has not been updated if the user pressed the Return key before the last actual keystroke.

Beware: I have no experience whatsoever in OS-X coding and Objective-C. However it does what I wanted it to do; see for yourself if it's implemented the 'right way'.