[6.0] diagram view (workflow) fails when labels contain non-ASCII characters or use m2o fields (e.g when showing group on wkf)

Reproduce by doing one of these with GTK client:
- change diagram view of workflow to display group_id, and notice how it fails as soon as you display a workflow diagram where a group_id is set on some transition
- or change one of the fields values that are used as labels and put some accented text in it, such as "ééé" and see how it fails to render the diagram

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I suggest to put the each element of the edge label on a separate line. And in the workflow diagram view, to display the group which is a very important business info in the workflow representation.
Merge proposal concerns:
client/bin/widget/view/diagram_gtk/parser.py : cleaning of the ugly code. Need to be merged before server improvement
server/bin/addons/base/ir/workflow/workflow_view.xml : display group
server/bin/addons/base/ir/ir_ui_view.py : improve label

Other improvement ideas are :
- do not define transition.condition as required with default value 'True'. The condition is not required. Making it not required will prevent polluting the diagram with "True" on all transitions without conditions.
- do not generate plenty of time the diagram when switching to diagram view, leaving it, or browsing. Generate it once, it should be enough.
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