[meta] New design of the API

[kkristof]

This issue tracks the state of the API refactoring, which was agreed during the GeCo 2017.

• Move the existing Gepard API under the new Canvas API
• Rename the GepardEngine to Gepard
• Streamline the Gepard API
• Expose the internal Gepard API
• Refactor the build system
• Update the examples to match with the new API