Group multiple date events

Problem/Motivation

To provides an extra plugin for the AddToCal module grouping multiple date events in one button.
This provides an extra display format plugin for the AddToCal module, allowing to display only one single button for events with multiple dates instead of one button per event date as in the original plugin.
The button for the event is also decoupled from the date field and now provided as an attribute to the entity 'addtocal_content_banner' and later printed on your custom template.