UpNextScrubberTouchbarItem
    stores update interval,
    stores settings of widget (interval duration, maxToShow, from, to)
    stores a list of 'IUpNextSource' classes that can source events from an application
    stores a list of 'UpNextItems' (actual buttons rendered to view)
    
    init()
        initialises and applies settings
        sets up IUpNextSources (classes that can source events from an application)
        sets up view of NSTouchBarItem
        
    getUpcomingEvents()
        iterates over list of IUpNextSources, calling their getUpcomingEvents() methods
    
    updateView()
        calls self.getUpcomingEvents()
        sorts the events by startDate
        create buttons for each event
        changes background color, binds tap to each button
        creates a new view, adds all the buttons and overrides existing scroll view
        resizes scroll view to new width

UpNextEventModel
    model passed to UpNextItem
    has a title (String), startDate (Date), sourceType (UpNextSourceType, where the event came from)

UpNextItem
    takes an UpNextEventModel and converts it to a button to be displayed
    has the rendering logic for each button element

UpNextSourceType
    is an enum that stores different source types, used by UpNextItem to know how to render itself and what action to take on the tapEvent

IUpNextSource
    interface for event sources
    
UpNextCalendarSource (implements IUpNextSource)
    manages getting Calendar events
    
    init(updateCallback)
        initialises dependencies
        stores the callback and adds an observer to EKEventStoreHasChanged to run the updateCallback
        
    getUpcomingEvents()
        takes a lower and upper bounds (calculated by "from" and "to"
        uses EventKitto get the events in this range
        formats them as UpNextEventModel (with an UpNextSourceType of iCalendar) and returns it