Remove an unnecessary allocation
Unify color parsing

Change utils_parse_color() to use gdk_color_parse() and follow its
syntax, additionally supporting our "0x" prefix as a synonym for the
"#" prefix;  and use this everywhere.

Also add utils_color_to_bgr() and utils_parse_color_to_bgr() to provide
conversion to the 24 bits BGR format used by Scintilla.
Remove useless utils_invert_color()
Remove useless rotate_rgb()