Modify AppKernel with plugin functionality

[2martens]

This task is about modifying the AppKernel class. The goal is to modify the registerBundles method in such a way that it is possible to dynamically change the list of registered bundles. The chosen solution for this purpose is a plugin directory. It contains an index file with an integer as the sole content and several other files which all start with an integer.

The AppKernel class must go through these other files in alphabetical order (that's why there are integers at the beginning) and create a bundle object from them. For this purpose the files will probably contain just the PHP statement that has to be executed (WIP). The created bundle object will be added to the existing bundle array. In the end it is returned in registerBundles.

This task contains a list of TODOs, each representing a logical sub exercise that can be tackled one at a time. They stand in the chronological order in which they should be worked on. In parentheses you can find the estimated net work amount required. This is a rough estimate and can be quite a bit off (in either direction). When you work on the issue, please update the text with the actual net work time you needed. This can then be used for improvements in upcoming estimations.

• think about location of plugin directory (estimated 0.5 hours amount of work)
• think about the content of the plugin files (estimated 1 hour amount of work)
• define public API of class (estimated 0.5 hours amount of work)
• implement class including test iterations (estimated 1 to 1.5 hours amount of work)
• write documentation in docs repository (estimated 1 hour amount of work)