[439871] Setup the CI-build with a specific headless profile
[439871] Remove UI dependencies which are not used *from the code*
[439871] Stop using LabelProvider so low in the Table dialect

EMF introduced IItemLabelProvider exactly for this need, to avoid 
depending on UI stuffs just to retrieve a label. This commit makes the
refresh use directly the IItemLabelProvider instead of wrapping it into
a useless labelprovider.
[439871] Add a default SelectorProvider

This provider is available when nothing else is contributed so that
the corresponding code will work nonetheless, but it is expected to be 
overrided by the one in diagram.ui.
[439871] Move the diagram refresh logic down to the non.ui plugin

Move the DiagramDialect and all the code it depends on in the non.ui
plugin.

Extract the ViewpointSelectionCallback logic in a dedicated class lower
in the stack (not in the UI) and delegating from the old one.

It is not compiling, we still have a reference to a
canonicalsynchronizerImpl and some logic to capture SWT line height.
[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).
[439871] Add extension pt to call the diagram.ui CanonicalSynchronizer

With this commit the CanonicalSynchronizer is called again when present.