Allow to set inital persistant value of COs

[daschuer]

This is a follow up of #534
It removes a hack for setting the initial persistent value, and allows whole Mixxx to benefit from it.

[daschuer]

Open discussion form: #534 (comment)

Please pull

@ywwg:

I'm not sure this is right, because there may be a difference between the default value (which is what you get when you reset the object) and the initial value. So I don't want to make this change yet.

Yes, you are right, there is one, even though in most cases a inital Mixxx start should move its persistent controls to the default value.
But in a rare case for a different value, it is perfectly OK to set a different default after the CO is constructed. This is the way Mixxx works in master. The default value is initialized with a fixed 0.0 and can be changed afterwards. This PR just adopts the initial persistent value as default this is slightly better than the random 0.0.

[ywwg]

The argument should probably be called defaultValue so people understand it's not just an initial value, and then the header comments should also say that the co will be initialized to the default value on construction.

[daschuer]

Names changed, comments added.

[ywwg]

inital -> initial

[ywwg]

Just some spelling fixes and a slight rewording of the comment, then LGTM

[rryan]

I'll do the fixes post merge since I'd like to get this into master before #411.