[FIX] props validation: does not crash with t-call-context

The code for props validation assumed that the rendering context was a
component.  This was actually true when it was written, but is no longer
true since t-call-context was introduced.

Because of that, it would crash when trying to access the internals of
the component, such as the static components object.

The fix is simple: instead of passing the context to the props
validation code, which can now be anything, we pass the component node,
which is guaranteed to give a reference to the component (and also to
the app).  This also make the code slightly simpler.

closes #1261

tests/components/__snapshots__/t_foreach.test.ts.snap

@@ -56,7 +56,7 @@ exports[`list of components crash on duplicate key in dev mode 1`] = `
       if (keys1.has(key1)) { throw new OwlError(\`Got duplicate key in t-foreach: \${key1}\`)}
       keys1.add(key1);
       const props1 = {};
-      helpers.validateProps(\`Child\`, props1, ctx);
+      helpers.validateProps(\`Child\`, props1, node);
       c_block1[i1] = withKey(comp1(props1, key + \`__1__\${key1}\`, node, this, null), key1);
     }
     return list(c_block1);