Configurable audio filter mode & power LED off-state

This adds a feature configuration command to Minimig for configurable audio filter mode & power LED off-state.

Audio filter: switchable - always on - always off
Power LED off: dim - off

With these options it is possible to configure several revisions of the Amiga:

Amiga	Power LED off	Audio filter
A1000	off	always on
A500 rev 3	off	always on
A500 rev 5	off	switchable
A500 rev 6/7/8	dim	switchable

(I don't know of the A2000 configurations...)

Additionally it's nice to have Audio filter always off option.

Now it is possible to enjoy some real old Amiga demos. Although the Soundtracker docs warned

[...]
Filter:  This Soundtracker version allows you to turn the lo-pass filter
-------  on/off.

Example: G#2 1E01 - Turns the filter and the power-led off.
         --- 0000
         E-1 1E00 - Turns the filter and the power-led on.

Warning to all Amiga 1000 owners:  Don't play with the power-led because A500
and A2000 owners will not be happy when they hear your song.
[...]

some A1000 demos used this feature.

Here are two examples if you want to try it out: https://workupload.com/file/3vXjChfVyeV

minimig-dev / MinimigAGA-MiST-TC64

Configurable audio filter mode & power LED off-state #3