Stabilization: get rid of AttitudePlus mode and always use trims

Having the trims object and an attitude mode that doesn't use them
is redundant. This now uses the trims in all modes, and to not use
them the value is set to zero (default).
Stabilization: drop the QUATERNION_STABILIZATION mode

This code is never used and has not been tested since all the new
modes were introduced. Lets delete it and reintroduce (and test)
it when or if desired. It also behaved strangely in simulation,
so may not have bugs.
Stabilization: minor refactoring for cleaner code

Now we always have trim angles simply put the setpoint in
those terms and use it for all the modes.
GCS: increment trim values when taking from transmitter

Now the system is always in the equivalent of previous "AttitudePlus"
mode, the trim should always be taken as a change in the setpoint
rather than an absolute value.
GCS: zero trim settings after leveling

After leveling the frame, rezero the trim settings. For most
people this will be the desired behavior and if there is a
residual trim offset they will probably wonder why it is not
flying level.
AttiudePlus: comments from review

Mostly just header file changes
Merge pull request #1107 from peabody124/default_to_attitudeplus

Default to AttitudePlus