fix issue with config/ini-file cal: entries and --cal

Fix issue where single --cal command line option doesn't override calendar-to-which-operations-apply as documented because of how the --cal command line option works.  This fix required a slight refactor to GetCalColors(), because it needed to construct the calendar -> color map without regard for any "calendar-to-which-operations-apply" constraints, but using any --cal command-line options' color overrides.  Apart from that the fix was straightforward: only work with the calendars specified on the command line if one or more --cal command line option was used.

Please can you take a look and apply if you're happy with it?