Feature preferred layout attributes

[zenangst]

Adds support for using preferred layout attributes on your cells.

[christoff-1992]

As mentioned, theres currently a bug with the width of the headers/footers on OSX. However this is present in master.

There is also a bug with the positioning of the cells when using dynamic heights for the header and footer, again this is present in master.

Example: The collection view is at its furthest position and the cell is overlapping.

Could this have been broken in #131

[christoff-1992]

Approved based on the comments (Pending tests been resolved, running locally without any issue), we will need to open issues for both items. I will also look at putting an example together for the changes, the example app needs updating for iOS 13 anyways.

[zenangst]

@christoff-1992 I'm gonna take a look at why the CI is failing, when that is fixed I think we should aim to get this into master to force us to fix all the small tiny things that might still be an issue. What do you reckon?