miniboxtk - no external dependencies

Miniboxtk does not have external dependencies except for the backend libs (e.g. libsdl2), which are all optional. This means there is no glib, boost, libxml2, python, etc. dependency. Which matters a lot because the application using miniboxtk also inherits all dependencies miniboxtk has.

Advantages of no external dependencies:

• smaller code footprint; we do not reimplement our external libs, we just don't have the excess code
• smaller installation footprint
• easier to package
• less risk of breaking or abandoned/obsolete dependencies