Plane: add param Q_TILT_BIAS_CH for RC control of motor tilt forward bias

[kd0aij]

supersedes #11270
this lets you assign an RC channel (knob or slider) to tilt the motors forward for higher speed in QHOVER, QSTABILIZE and QACRO modes; pitch stick controls attitude as before
Has no effect in QAUTOTUNE mode or in FW modes with the motors fully forward.

flight tested In SITL and with PixRacer/Convergence

[kd0aij]

low speed QSTABILIZE mode flight test looks good. RC8 is the tilt channel:

higher speed test (flown into the wind) including QHOVER mode; but do we want to allow this to affect QHOVER?

[tridge]

I think this could be done a lot more simply by modifying tiltrotor_slew() to add to the slew

[kd0aij]

@tridge I don't see how the logic can be simplified by moving it down into tiltrotor_slew(), but I rearranged it to make things less confusing. Also moved the check for channel assignment out of the main loop and into init code.

[kd0aij]

E-Flite Convergence test flights: modes QHOVER and FBWA. Wind speeds 5-15 m/sec.
QLOITER mode seemed unstable when headed downwind. QHOVER mode behaved pretty well using tilt bias to compensate for wind.

[kd0aij]

More flight testing with the Convergence; with tilt_max=25 degrees, top speed is 10-15 m/sec in Qmodes, and handling is good. Transitions to/from FBWA and QLOITER were fine at min and max tilt.