🎨: add support for reverse ordering in layouts

[merryman]

This is a suggested enhancement to Tiling Layouts which I found quite useful when playing around with the implementation of the history page of the lively.next website.

Since we are not able to vary the structure of morphs via master styling easily, I found it helpful for the Tiling Layout to support a property that basically allows us to reverse the order.

For demonstration purposes, I simply used the already existing (but not really useful anymore) orderByIndex property and deemed it to denote wether or not the morphs are laid out in reverse or not.

If this is useful, I would consider renaming the property and enhancing the sidebar controls to support this config.