Introduce a maximum cut-depth to strategies and a strategy to gradually increase these.

This should allow to do roughing = cutting in layers (in user-defined layer-heights) until a skin of a user-defined thickness remains.

Added support for multiple strategies (with different tools). No layer-support for roughing yet.

Added the UI elements required (still disabled)

implemented roughing with layers (no skin thickness yet)

• Allow 2 user selected feed rates (jogging, cutting)
• Allow a skin-thickness for roughing