Decouple workflow task schema from layout

Goal is to be able to build a task variables schema without having to define its layout.

• add ability to select global variables on the variable tab
• Make sure it is still possible to define the buttons to close the task, but call it "resolution actions"
  See mockups.

Since some people are still using the JSF UI, and since it is theoretically possible to use the JSF UI and the web ui at the same time, we could work with a sort of "version" on instances of workflow model.:

• model created without version / flag : display same tabs as before and use the former logic for generating the xsd schema.
• model with the version / flag (actually all new instances) : display tabs ni mockup + form tab if jsf is enabled (when it will be enabled), and generate the jsf form if jsf is enabled.