Add support for `ILayoutElement` properties

[RichardWepnerPD]

The ILayoutElement defines some properties that are used by other components in UGUI and thus vital for defining UIs using UGUI. These are min, preferred, and flexible width and height.

One example use case would be a ScrollRect containing content layed out using the FlexLayout. Since the content element of the scroll rect needs to have a size in order for the scorlling to be limited accordingly, a ContentSizeFitter is frequently used to restrict the size of the element to the preferred height (or width).

Adressing this might also resolve the issue #24.

[gilzoide]

I'm closing this issue and focusing this discussion in #27, which addresses this topic and more.