Create a C++ plugin module

[iBelieve]

I've wanted to avoid doing this to keep the library as simple as possible, but I think we'll have some significant improvements from doing so.

• Calculate dp correctly and on a per-screen basis (Text size is too small in Windows #230)
• Possibly add image providers ([RFC] Create a Qml ImageProvider plugin for icons. #336)
• Handle anything else that should best be done in C++ (for performance reasons, etc) (Extras.Image blocks UI when calculating averageColor #337)

Want to back this issue? Post a bounty on it! We accept bounties via Bountysource.