Enable devices to define their default themable icon.

Kit icons are now by default the Desktopdevice icon,
overridable by the device icon of the kit's device,
in turn overridable by an icon file path.

KitManagerConfigWidget: The browse button for the Kit
icon gets a "reset" action.

Change-Id: I6328ba7d640393aee2c324c592e76b4d5430586a
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>