LsColors: change has_color_for to public visibility & rename to has_explicit_syle_for
coreutils (https://github.com/uutils/coreutils) uses style_for_indicator
to check if a style for Indicator::Capabilities is available.
However, style_for_indicator uses fallbacks and can therefore return a
style even if no direct style for Indicator::Capabilities is set.
Directly being able to use has_color_for allows to fix that.