WIP: Pure Attribute/Widget definitions

[TimLariviere]

Removed the auto-incremented key and the need for AttributeDefinitionStore and WidgetDefinitionStore. Now their keys are their names, so all definitions require a unique name.

Attributes and widgets now take a direct reference to their definitions.

I had to change a lot of stuff to avoid having everything interdependent (type ... and ... and ... and ...). But I feel most of my changes make even more sense than before. So I'm quite happy.

Only ViewNode is currently very loosely defined. I have to properly think this one.

For now only the project Fabulous compiles