Fine-grained pin compatibility checking

[micheljansen]

Limit not just by type (OpenCVImage<->OpenCVImage) but also by properties (depth etc.).
Challenge: come up with solution that does not just work for OpenCV datatypes, but also for those added in the future.

One solution might be a mime-like compatibility list like Qt's drag&drop framework uses: http://doc.qt.nokia.com/4.6/qmimedata.html