ComboBox: can't customize popup palette

See attached.

While it is indeed possible to customize a ComboBox by redefining its items, sometimes a quick solution is just to act on palette colors.

For example, to achieve a dark theme, one could override palette.base and palette.text.

However, the combo popup is not affected at all. See Screenshot_20181228_145719.

Patch proposed. See screenshot: patched_control