Add SizeMode enum that determines what sizing rules an element uses #39

Something like this, where Auto is the default and uses the current value-based behavior:

public enum SizeMode {
    Auto,
    Absolute,
    PercentageOfParent,
    PercentageOfSelf
}

There should probably be independent values for the X and Y axes so that this makes sense (since the current behavior allows independent X and Y axes as well)