Customizer: Iris color picker handle above section title

[crstnio]

When Iris color picker is opened, the handle displays bevor the section title – if section is visible during scroll-up.

Section title styles:

#customize-controls .customize-info.is-in-view,
#customize-controls .customize-section-title.is-in-view {
    z-index: 9;
}

Handle styles:

.iris-picker .iris-square-handle {
    z-index: 10;
}

[crstnio]

Iris color picker handle z-index bug

[dlh]

#47649 was marked as a duplicate.

[dlh]

This might be as simple a fix as boosting the section title z-index, but it'll definitely need testing.