[WIP] Make popovers more customizable

I'd like to make popovers more customizable. Specifically, I'd like to add easy customization of the size and position of the arrow. This is difficult with the current interface for a combination of reasons:

* The current API tries to build the customization into `RBLPopoverBackgroundView`. But since the actual view isn't created until you show the popover, there's not a suitable point where you can configure the view.
* The methods that convert content sizes and frames to window sizes and frames are class methods, so their values can't be affected by properties.

I'd like to move the relevant properties and methods to `RBLPopover`. It's no harder to subclass than the view, and you can do almost all of the same things, but it'd also be easier to add customizable properties to `RBLPopover`.

This PR just changes the header at the moment because I want to get some :+1:s before I do the work.