Colour config doesn't get applied in Wayland

[Dave Shoreman]

Where Slop takes one colour option, Slurp takes three and they're in a different format too.

Slop uses decimal RGBA whereas Slurp uses Hex, so we'll need to add some translation layer (or duplicate the config option) to allow for a different selection colour in Wayland.

While not documented in slurp -h, Slurp has the following defaults:

    RRGGBBAA
-b #FFFFFF40    Background colour
-c #000000FF    Border colour
-s #00000000    Selection colour