Use the text foreground color for icons when the theme doesn’t provide a color and introduce ‘gutterSelectionIcons’ for the color of selected icons (defaults to selected foreground color).
Also changed the gutter selection border color to match the divider color (when it isn’t specified by the theme).
Previously if the color for X (where X is e.g. foreground or invisibles) had an alpha value other than 1.0 (fully opaque) it would be blended against the less specifically scoped color for X rather than the background.
There was no real logic behind the previous design other than to treat all color blending the same and it would cause a problem for 1.x themes using transparency for selection or invisible characters.
Closes issue #285.
