Determine data paths at runtime

Allow override of datafile paths with $GRAMPS_DATA

Write only the installation "share" path into const.py
Test both the overridden path and the configured path for authors.xml,
raise an EnvironmentError if it's not there.
Set the actual variables (DATA_DIR, etc.) with os.path.join and
SHARE_DIR.
Replace installation icons/hicolor with images

Because icons/hicolor is a leftover from Gtk1, and we're on Gtk3 now.