416 Make a Better Plugin Manager

https://github.com/Freeseer/freeseer/issues/416

This is a start to solving the issue. There are some dummy plugin modules set up, as well as a start to a "ModuleManager".

[Project proposal](https://docs.google.com/document/d/1ll7hGwjPzD6HLJ79WRGduJyUAprY4RU9REZznX8iVMQ/edit?usp=sharing)