Update behavior properties without setAttribute about scrollmeister

[Prinzhorn]

Problem: this will trigger attributeChangedCallback and parse the whole thing again. For some prop types such as LayoutDependencyType this might have performance implications. Instead we could just update the internal representation like a-frame does. In debug mode we can then force-sync the attribute, which will cause a full re-parse. a-frame uses a flushToDOM method.

For now we're not syncing the values back to the attribute, just interally.

The tricky part is not everything can trivially be serialized back, e.g. LayoutDependencyType. We might have to store the original value (e.g. selector or 'skip 2') attached to the array.

LayoutDependencyType now also returns the original value so we can easily stringify (which the schemaParser can do now and is also tested)

[Prinzhorn]

64e90cc