Allow theme to design how to draw window icon in window content when window is unmapped (window content is unavaiable)

Add themable properties to XfdashboardLiveWindows which control how the window icon is drawn in window content when window is unmapped due to window manager limitations, e.g. window is minized or window is not at active workspace when using xfwm4. The properties are prefixed with "unmapped-window-icon-".