Since the tune library also contains logic how tunes can be overriden,
a user of the tunes library cannot know the strength of the current tune
without replicating some logic. Easy solution is to add strength to the
output of Tunes::get_next_tune().

Otherwise the only way of interrupting a repeated tune would be with the override flag, which should only be used rarely.
Now repeating tunes have lowest priority, followed by one-shot tunes, followed by anything with an override flag.

Override flag must still be set to true!

Bugfix: This fixes a bug where libtunes might indicate that there are more tones to play even after the last note of a tune.

Custom msg tune can override any tune playing, regardless of the tune_override flag.

Revert the disabled d term filter which needs to be enabled
after reducing IMU filtering and was probably unintentionally
during a rebase.

 - fixes coverity CID 196758

 - fixes coverity CID 260383

 - fixes coverity CID 264261

 - fixes coverity CID 264263

 - fixes coverity CID 264259

Signed-off-by: Roman <bapstroman@gmail.com>

If not in air-mode the mixer is not able to apply positive boosting
and roll_pitch_scale is recomputed to apply symmetric - reduced - thrust.
This has the consequence to cut completely the outputs when the thrust is
set to zero.

this avoids calling param_get() on every loop iteration.

Added in 262d9c79.

- fmu: in case of _mot_t_max==0 _airmode was not set in the mixer
- px4io: param_val is a float