Have "gtk/color_utils.c" contain routines with toolkit-independent APIs,

Have "gtk/color_utils.c" contain routines with toolkit-independent APIs,
but toolkit-dependent implementations, for manipulating colors, and have
"gtk/color_utils.h" declare them (the header file should eventually be
moved to the top-level directory).  Move the routines to convert between
GdkColor and color_t out of there into "colors.c", and move their
declarations into "colors.h", as their APIs are toolkit-dependent.

Have the first such routine be a "create_color()" routine, which takes
RGB values and initializes a "color_t", including doing any
toolkit-dependent work necessary for that; use that in the
"gtk/color_filters.c" code (the goal is to remove as many of the toolkit
dependencies as possible from that code, and move it to the top-level
directory).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@11497 f5534014-38df-0310-8fa8-9805f1628bb7