[WIP] Make popovers more customizable

[mdiep]

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.

[jspahrsummers]

This makes sense to me, though I think @dannygreg should take a look.

[dannygreg]

It's no harder to subclass than the view

Not true. How do I do custom drawing? RBLPopover isn't a view so that becomes incredibly unclear. I think the answer here is to change how we are initialising the background view. I quite like having the drawing responsibility as clear as it is now.

[mdiep]

What kind of custom drawing do you have in mind?

[dannygreg]

What kind of custom drawing do you have in mind?

A different style of popover altogether. After all, that was the reason we created RBLPopover in the first place.