Refactor: Reasonable model methods and properties are made static

[JasonTheAdams]

Many of the properties and methods of the Model class are instance properties for no particular reason. The Model::$properties property, for example, is an instance property. But it's really only meant to be defined by the class and then consistent across all subclass instances. In fact, making the properties dynamic on a per-instance basis is an anti-pattern to the model itself, as it's no longer a clearly defined set of data.

The side-effect of these being instance bound is that an instance is required in order to do basic things such as know a model's properties, check if a property is valid, and so forth.

This PR switches a number of properties and methods to be static. Basically those that can be. It updates the interfaces and the Model class itself.

Accessing a static method from an instance (e.g. $model->hasProperty() is supported in PHP and backwards compatible, but the interfaces and property/method overloading obviously aren't. As such, this is a major change and slated for a 2.0.0 release.

To do in this PR:

• Update the docs