Warn if we are overriding is= attribute

[lourot]

When doing

<ImageDisplayControl>
  <img src=... />
</ImageDisplayControl>

an is="image-display-control" attribute will be added to the <img> tag in order to turn it into an Image Display Control web component. This is problematic however if the <img> tag already had another is= attribute, e.g.

<ImageDisplayControl>
  <img is="already-another-component" src=... />
</ImageDisplayControl>

An <img> tag can't be two web components at the same time. We should at least issue a warning if this happens.