[439871] Use CanonicalSynchronizer to hook diagram header computation

This commit adapt the CanonicalSynchronizer API so that it wrapps the 
computation of the diagram header (which can only be done with an UI).

Change-Id: I6250c5a628f76c4d038cf248851b1e06c01a0534
[439871] Add extension pt to call the diagram.ui CanonicalSynchronizer

With this commit the CanonicalSynchronizer is called again when present.

Change-Id: Ic984c9dbba3208a0afb11820f1c05fc6e3756aaa