Snapshot enhancements

[skgrush]

Encapsulated all the snapshot logic into a view extension and a single update check. To implement snapshotting, a view now just needs to

• have an @ObservedObject var snapshotModel: SnapshotExportModel
• attach to the view .snapshotMenus(for: _snapshotModel)
• include in the view snapshotModel.promptButton(scan: scan) wherever the button should be included
• in the render controller update method call snapshotModel.viewUpdaterHandler(scnView: uiView, overlay: X) with an optional overlay SKScene.

When the prompt button is clicked, an action sheet appears with scaling options:

Once a scale is chosen, the viewUpdateHandler triggers a PNG render of the given scene (with optional overlay) scaled with the selected multiplier, then opens the PNG in a share sheet.

[skgrush]

This will resolve the 3rd item "snapshot tool action sheet" from #17.