gui: Simplify color handling.

Various gui elements of para_gui can be customized through struct
gui_theme. Currently this structure contains two integers for each
each element, one for the foreground and one for the background color
of the element. This is a bit clumsy and repetitive.

This commit cuts down the number of fields of struct gui_theme by
a factor of two by introducing struct gui_color_spec as a pair of
integers.

The patch is quite large but also simple.