build: Remove all 'tracker' uses in configure.ac.
extract: Port Qt backend to Qt5 and NEMO platform

Commits squashed and based on tracker commits:
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8b3b8fe Fixes after review for Qt5 port
934eb10 Use minimal in Nemo for QGuiApplication for in case the compositor isn't running yet. Use QGuiApplication instead of QCoreApplicati
601be18 Port to Qt5.
8dfadf3 tracker-extract: Fix compile of Qt media art backend
build: Fix selection between backends Qt and GdkPixbuf
extract: Fixed Qt(4) backend to compile and removed all Tracker references
docs: Remove reference to Tracker in reference overview