Refactor control add_item, allow section overrides

By refactoring the logic to construct the args passed into `$wp_customize->add_setting`
and `$wp_customize->add_control`, less duplicated logic is in the controls.
With the control args array now constructed in one place, this gives the
opportunity to introduce a new element field 'section' that can override
the default group that a control is assigned to. This is ideal if you want
the styles controls to be located within existing sections (which I do).