Smooth camera movement

• Add an acceleration factor to settings that smooths the camera movement.
• Let movement speed ramp up over time when button is pressed, let it ramp down when button is released
• Keep momentum when changing directions
• Also for orientation of camera / view axis / forward vector
• Default should be similar to current behavior, i.e. high acceleration / deceleration (instantaneous max speed / stop)
• For now, only active in playful mode