Soft takeover fixes

* sync soft takeover on effect load
* double the takeover threshold for half-linked parameters
better place for comment
herp derp
Superknob soft-takeover issues -- Address notes and add test.
Fix test
typo
Fix EQ selections incorrect when deck count changes from 2 to 4

Fixes lp:#1404808
Auto DJ: Create a simple bandaid for the case when decks 3 and 4 are playing.

This is not pretty, but we already do something similar when decks 1 and 2
are both playing, and we are out of time to design a nice way to handle a
"4-deck" autodj.

This will eventually be replaced with something nicer!

Also fixed the autodj button not getting un-pushed when the enabling failed.
Remove -6dB gain in EnginePregain. Fixes Bug #1404985.
Change replaygain boost range to allow negative values.
After scratching, sync phase if follower and quantized.
Added plausibility check for m_callbackEntrytoDac

If it is invalid, a one time warning is logged.
This is an attempt to fix Bug #1404803
fix EffectParameterSlot::syncSofttakeover()
Sync superbutton softtakeover after force write
VisualPlayposition: Move warning up
Merge branch 'master' into softtakeover-fix
remove syncsofttakeover call
Merge pull request #434 from ywwg/softtakeover-fix

Soft takeover fixes
Deere: Disable deck sync_master buttons since they are not supported fully
Migrate pre-1.12 replaygain values on upgrade by subtracting 6.