Add better facility for using eureka from remote site.

[benoitg]

This is building on the idea of the current feature where the Eureka metadata editor can be used from a remote site in an integrated way.

Similarily, thematic navigation and search could be integrated.

Tis will require the following:

• Allow the remote site to disable the left menu
• Allow the remote site to trigger having a back link in case the interface isn't displayed inside a IFrame
• Optionally, allow the remote site to provide a custom stylesheet.