fix #770 Multiple embed:Elements linked to page engine instances cause w...

...idget errors

This PR fixes the first part of the issue.

For the second part, it is much more complicated.
By design, placeholders are currently managed globally and so their id must be unique in a given application.
It would possible to change it, but it would have many impacts on the framework.
So for now, this won't be changed as a workaround is available.