tracker-preferences: improvements to the ignored_dirs gtkentry UI
tracker-preferences: improvements to the ignored_dirs gtkentry UI

* removes the separator between text entry and the 2 buttons
* OK is the default action when hitting return

these were suggested by mitch

hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker