menuconfig: Add support for custom color schemes (aka styles)

- introduce the _parse_styles() function which can take a string
    defintion and parse it into a color structure that can be used
    by UI elements

- the _parse_styles() function modifies a global dictionary (_style)
    which is subsequentially used to style the UI elements

- the _style_to_curses() function takes a string representation
    of style colors and converts it to ncurses attributes.
    For eaxmple: fg:brightred,bg:black,bold -> returns a color
    pair representing brightred and black ORed with A_BOLD

- simplify _init_styles() as now the majority of the heavy lifting
    is done by _parse_styles(). _init_styles() initializes the
    default theme (either "defualt" or "monochrome")

- terminals without color capabilities are now handled by using
    the "monochrome" theme. This theme doesn't use any colors
    but rather just bold and standout attribtes.

- add support for the "underline" attribute

- add doctstring at the beginning describing the style syntax

- rename the _style() function to _style_attr() as it was conflicting
    with the global _style dictionary

- introduce the StyleError exception; this one is thrown if an error
    in the style syntax is encountered

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