Model behaviors become part of the core framework

[mariuswilms]

Behaviors play an important role in the extensibility of the framework. Helpers are another mechanism that do so. In contrast to helpers, behaviors have been developed separately from the core framework combined with the hope to find the best solution.

I think it is now time to pick this solution.

Behaviors will become part of the framework, practically merging li3_behaviors from https://github.com/UnionOfRAD/li3_behaviors.

• copy implementation over from li3_behaviors into li3's 2.0 branch, keeping the namespacing from the plugin
• fix any deprecation warnings/errors while making behaviors compatible with 2.0
• add tests (the implementation inside the plugin changed but their tests never did)
• move the documentation from the readme of the plugin into a manual page, preferably under the models chapter https://github.com/UnionOfRAD/manual/tree/master/05_models