[qtdocgallery] Fix DocumentGallery returning encoded urls.

Tracker stores url properties in encoded form, these need to be
decoded before they are returned and encoded when used in queries.

Cherry pick of c4a36f94bd50d0236aaa7bddbe72bc7e0fbdac4f from
qt-mobility.

Change-Id: I26db02e758cb14e6789418631979fa21cf58daff
Reviewed-by: Marko Mattila <marko.mattila@jollamobile.com>
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>