add AutoDJ xfader recenter option (default off)

[ronso0]

adopting #13156 but cleaning up the "buffer" settings before (that dlg wrote some "...Buff" settings to config for storing temp pending settings...)

Fixes #11571

TODO:

• [x] add a note how re-centering may affect main volume (see #10683 and #4714)

[Eve00000]

Hi Ronso, I was almost ready to upload my adapted version, but you already did the changes. You cleaned up the whole routine. Thank you for the work and congratulations it surely will be better then mine . I will learn by comparing your version with what I did.

If it's possible I just want to add the lines in the ui with some explanation (tooltip), I don't know how to add that in the pr so I'll write it here.

<property name="toolTip">
<string>When checked, the Crossfader will be set to the default center position after disabling the Auto DJ.
This can be handy when you're working with an external mixer and don't need to see the Mixxx-mixer and crossfader 
(mixer and/or crossfader disabled in the skin-options).

Please mind that the crossfader adds some volume to the pointed playing deck in the main output, 
so centering the crossfader will cause a volumedrop in the main output.
</string>
</property>

greetings

[ronso0]

Thanks for your proposal, I already added the hint as static label (no as tooltip which may not be discovered).

Ready for review!

[Swiftb0y]

Sorry, forgot about this.