TrackerDecorator: Fix a compilator warning
TrackerDecorator: Fix a compilator warning

gcc was thinking that strings could be used uninitialized.
This fix the warning and simplify the code using a GPtrArray
which is more natural for string arrays.

https://bugzilla.gnome.org/show_bug.cgi?id=719802

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