Twenty Fifteen: Move the custom Customizer class to its own file.

[kwight]

Let's separate out the control class to its own file, so its treated more like core treats classes. This also simplifies customizer.php and removes the need for altering hook priorities.

[obenland]

It should become obsolete once the scripts are moved.

cainm said this:

you can get rid of the custom control and then enqueue the js on the customize_controls_enqueue_scripts action
but I didn't have time to narrow down the control extension past: wp.customize.controlConstructor.select

[kwight]

Yay! How quickly I forgot about that :)

[cainm]

Related: #29982

[iandstewart]

In 29944:

Twenty Fifteen: Removing unused custom color controls.

Props cainm, fixes #29982 and #29959.