Add a "color.h" file that declares a nominally-toolkit-independent

Add a "color.h" file that declares a nominally-toolkit-independent
"color_t" structure to store color values (although currently it has all
the same fields that a GdkColor has; its currently advantage is that you
don't have to include any GTK/GDK stuff to declare it).

Add routines in the "gtk" directory to convert between "color_t" and
GdkColor values.

Define, in "prefs.h", all colors as "color_t" values rather than
GdkColor values.  "prefs.h" now no longer needs to include <gtk/gtk.h>,
so don't include it.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@2692 f5534014-38df-0310-8fa8-9805f1628bb7