Set GC_DEBUG for debug builds

.. when building a debug build of GC with CONFIG += debug
   then we define a macros GC_DEBUG.

.. main no longer redirects to goldencheetah.log if you have
   GC_DEBUG set, so developers will continue to see debug
   messages on the terminal output (for mac/linux only).

.. we could use the GC_DEBUG macros to allow debug messages
   to be switched on / off in the code in future
Compare Pane Mechanics

.. compare pane now appears and disappears on both
   the analysis view and home view

.. it also is notified of drag/drop events and can
   accept the objects dropped

.. it is now time to write the widget for collecting
   and managing compare sets