Refactor filters

[yeahphil]

• Add a Filters struct with same properties as the filtering bits of DefaultsManager
• Extract predicate generation from RunsListViewController to Filters
• Remove predicate stuff from MapViewController, use Filters instead
• Make a static default filter to set on first run
• Use DefaultsManager.shared.filters to manipulate filters in OnboardLocationViewController
• Maintain FilterViewController state while editing in a Filters var, and save it on dismiss
• Mark backing store of filter properties in DefaultsManager private
• More refactor of FilterViewController
• Reorganize FilterViewController file
• Use xmark.circle (SF Symbol) for clear region filters button in FilterViewController
• Not necessary to reload the filter controller's collection view every time the distance slider changes
• Post a notification when DefaultsManager.filters change
• Change map view fetched results controller lifecycle:
• Another try: show loading cell or 'no rivers available' cell as appropriate