This patch allows a widget to not specify any icon
but it still enforce when an icon is specified to
specify only one icon and to provide the attribute
"src".
Bug-AGL: SPEC-702
Change-Id: I916b7581ac1f486a4a1d5774d0885c3711029f69 Signed-off-by: José Bollo <jose.bollo@iot.bzh>