Add a content_width helper (similar to content_height)

For horizontally-scrolling views, a `content_width` helper would be pretty useful. I added it, and made both `content_width` and `content_height` delegate to a `content_size` method instead to avoid duplicated code.

I also refactored the calculations a bit (mapping all of the subviews' dimensions and pulling the max value).

If anyone has a better name than `content_size` (which might easily be confused with `contentSize`), I can definitely change it. Maybe `content_dimension`?