App icons don't consider disabled state

[walterra]

Actions for EuiInMemoryTable support icons and when an action is not enabled the icon and text will be grayed out.

When using an icon from the "Apps" section (https://elastic.github.io/eui/#/display/icons) the icon won't be grayed out though, it looks like the colors are hard coded for the icon and the action is not able to override the style.

In the example below you can see that the icon for "Start" is correctly grayed out, while the icon for discoverApp is still colored: