[Search] Partial fix for bug #38

- Removed a trailing space in front of the search string.  This
  was causing problems for minidlna.
- dleyna-control no longer issues a search if the search string is
  empty, i.e., images, audio and video are unselected.  The old code
  was causing an invalid query to be sent which was generating an error
  in rygel can causing weird behaviour in minidlna.

Signed-off-by: Mark Ryan <mark.d.ryan@intel.com>