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: Ieeb17e4de8885a3a525b35ae365e51b421d39f83 Signed-off-by: José Bollo <jose.bollo@iot.bzh>