tracker-extract,gstreamer: don't compile both tagreadbin and decodebin2
tracker-extract,gstreamer: don't compile both tagreadbin and decodebin2

... and prepare for the Discoverer API based implementation

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