- Implement patchbay node search functionality

This adds a QLineEdit which appears when typing in the canvas, selecting nodes that start with or contain the typed text.

A namespace was added for future enum-based settings, to register them in Qt's meta-object system. This aims at type safe serialization by storing strings instead of integers whose order may change.

If you think this change is worthwhile, I will convert the other enum settings to use it as well.