Allow looking up window system colors on Haiku

Allow looking up window system colors on Haiku

* lisp/help-fns.el (help-fns--editable-variable): Fix describing
variables which don't have symbol values.
* lisp/term/haiku-win.el (haiku-allowed-ui-colors): Set list of
allowed UI colors.
* src/haiku_support.cc (be_get_ui_color): New function.
* src/haiku_support.h: Update prototypes.
* src/haikufns.c (haiku_get_color): Look for defined UI color.
(syms_of_haikufns): New defvar `haiku-allowed-ui-colors'.
* src/haikuterm.c (haiku_term_init): Fix coding style.