Mixer throttle fixes/improvements + more OSD Throttle changes

[breadoven]

Refactors parts of the mixer throttle code to clean up unnecessary calls to getThrottleIdleValue function. Also, improves throttle motor setting when disarmed or motor stopped.

For the OSD throttle percent for the scaled option is now based on mixerThrottleCommand rather than rcCommand[throttle] since this better reflects what is actually sent to the motors. The OSD scaled throttle option now shows "IDLE", "STOP" and "DARM" rather than just "0" to make it clearer what the motor is actually doing. OSD change also remove the Scale symbol before the Throttle symbol for the scaled throttle field since it doesn't seem necessary given the additional messages make it clear this is the Scale option. Happy to reinstate but always thought it looked a bit odd and out of place and out of proportion to the Throttle symbol itself.

[MrD-RC]

IMHO, the whole throttle system still needs sorting out. Scaling is happening in the wrong place. Everything up until the point of output should be based on 1000-2000us throttle. This will keep everything nicely in line. It only needs to be scaled when outputting to the ESC(s), or the scaled OSD element. At no other time should anything care about scaling. All navigation etc should be based on the 1000-2000 range.

[DzikuVx]

There was an attempt to change scaling of everything into [-1:1] but it was never finished. That would be the best solution