Rewrite plugin system

[jaccomeijer]

With the current plugin system plugins register themselves independent of other plugins.

The main disadvantage of this approach is that each plugin registration causes a DOM render.

We have to register plugins differently. Plugins can be imported and they the register call can pass all the plugins as an array.

Bug #117 can then be solved easily by checking the pre-renderend theme. If the pre-rendered theme is different from the theme that should be rendered based on OS UI color settings, render the correct theme instead.