* app/channel.c: get rid of compiler warnings

        * app/histogram_tool.c
        * app/histogram_tool.h: added a gradient below the histogram so
        it looks more like the curves and levels tool

        * app/gimpbrushpipe.c
        * libgimp/parasiteio.c
        * libgimp/parasiteio.h: plugged another mem-leaks. At least
        The GIMP now starts without leaking memory...

--Sven