Allow specifying icon colors toolbar commands

#### Is your feature request related to a problem? Please describe

In this particular case, ui.command items added into the toolbar card represents some state. So ability to provide different colors to the icons based on state will ui improve the user experience of the application. Also another use case
would be to choose icon colour according to the app theme.

#### Describe the solution you'd like

Extend the ui.command API to expect an icon color.