Redraw Now button & select-image button highlighting

• Added a “Redraw Now” button in Main.storyboard.
	• Triggers `- redrawNow:` in ViewController, which (via `- redrawDestImage`) handles setting the `_destImage` and `_imageView.image` to nil.
		» `- pointsDidChange` now uses redrawDestImage as well.
• Fixed select-image button’s highlight state reverting as soon as tapped (instead of when the menu closes) by making ViewController a QBPopupMenuDelegate and setting the button’s `highlighted` and `selected` to NO on `popupMenuWillDisappear:`
	» `- selectImage:` now sets the `highlighted` and `selected` states to YES, via animations (to avoid being overridden by built-in UIButton logic, and to look nice).
• Minor: Removed unnecessary clear of `_srcWidth` and `_srcHeight` in `- switchToImageNamed:`.
• Minor: Changed select-image button’s initial text in Main.storyboard to be more-clearly-placeholder “{image-name}”.