AP_Mount: New MNT_SMOOTH_TILT parameter

Adds a new MNT_SMOOTH_TILT parameter that can apply a smoothing factor
to the RC-input of the mount tilt/pitch movement.  This is useful in
setups where an RC channel (i.e., via headtracker) is used to control
tilt on separate cameras; one for FPV-flight and one for recording (on
the tilt-mount).  The tilt on the recording camera will follow the FPV
camera, but slowed down via the smoothing factor (and a bit of
deadband).  Setting of 0 (default) is no smoothing; 50 gives a good
"average" amount of smoothing; maximum value is 255.