Some icons are barely visible in dark theme

On KDE desktop, some QtCreator's icons are barely visible when:

1. The theme of desktop is light(e.g. breeze)
2. The theme of QtCreator is dark(e.g. flat-dark)

This is because QtCreator always uses current system icon theme.

Howerver, some people use the dark QtCreator theme while keep the global desktop theme in light.

(Because KDE's breeze-dark theme is not good enough and some apps are ugly under dark theme)

I think the simplest solution is adding a command line option from which people can select the icon theme, such as:

qtcreator -icontheme breeze-dark

A simple patch has attatched: icontheme.patch