Enhancement: Marketplace

*Warning: This pullrequest is only opened for beta testing.*

What does this change? It resolves #497.

Currently, this pull should *not* be merged, but I would be happy if someone else could also beta test it and share me its opinion. Please assume this as a beta stage. Functionallity should be mainly given.

- Combine Theme and Plugin Manager to one single component
- New Layout for displaying plugins/themes (Support for screenshots)
- Included 24h cache, so market server will be only triggered each 24h
- New Plugins can be announced in the component
- Installations will be tracked to calculate usage and popularity of a plugin/theme

*Note: Code is not structured nor commented. So it does not make sense to review the code behind.*