[IMP] component: allow dynamic root nodes

closes #283

tests/component/__snapshots__/props_validation.test.ts.snap

@@ -11,9 +11,11 @@ exports[`props validation props are validated in dev mode (code snapshot) 1`] =
     let QWeb = this.constructor;
     let parent = context;
     let owner = context;
+    let result;
     var h = this.h;
     let c1 = [], p1 = {key:1};
     var vn1 = h('div', p1, c1);
+    result = vn1;
     //COMPONENT
     let def3;
     let w4 = 4 in parent.__owl__.cmap ? parent.__owl__.children[parent.__owl__.cmap[4]] : false;
@@ -43,6 +45,6 @@ exports[`props validation props are validated in dev mode (code snapshot) 1`] =
         def3 = def3.then(()=>{if (w4.__owl__.isDestroyed) {return};let pvnode=w4.__owl__.pvnode;c1[_2_index]=pvnode;});
     }
     extra.promises.push(def3);
-    return vn1;
+    return result;
 }"
 `;