added popTo(item:orDismiss:)

[eliburke]

Checklist

• I've tested my changes.
• I've read the Contribution Guidelines.
• I've updated the documentation if necessary.

Motivation and Context

This is loosely related to my issue #103. It resolves a situation where a Page can be used either standalone or as part of a combined workflow.
e.g. a pair of Pages used to Select an Album and Select an Image in an Album could be used:
AppUI -> Send Image -> SelectAlbumPage -> SelectImagePage -> action dismisses
vs
AppUI -> ViewAccountPage -> EditAccountPage -> Edit Avatar -> SelectAlbumPage -> SelectImagePage -> action should pop to EditAccountPage

A page constructor can be created that takes an optional "intermediate root":

    @objc func cancelTapped(sender: UIButton) {
        if let myRoot = myRoot {
            self.manager?.popTo(item: myRoot, orDismiss: true)
        } else {
            self.manager?.dismissBulletin(animated: true)
        }
    }

Description

Created new popTo(item:orDismiss:) method. It takes an item and searches the internal itemsStack for an exact match to the supplied item. If the item is found, it is set as current and redraw is triggered. Subsequent items on the stack are then torn down.