Fix PlasmaCalendar widget to not mark days with minor events

Summary:
CalendarEvents::EventData (in KDeclarative) has a property isMinor, which is
described as "If set to true, it won't be marked in the calendar grid".

This patch fulfils that promise, by introducing separate roles on the model
which explicitly report the existance of any major or any minor events at a
given day, so that the marker loader can concentrate on the major events.

Test Plan:
Using a plasmacalendar plugin which feeds events with isMinor=true the
widget now only marks days which have major events.

Reviewers: Plasma, broulik, davidedmundson

Reviewed By: Plasma, broulik, davidedmundson

Subscribers: davidedmundson, broulik, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12438