Fix scaffold of task variables in workflow layouts

Dates in workflow default layouts are not completely "human-readable" (see screenshot).
Currently, we have hardcoded in https://github.com/nuxeo/nuxeo-studio/blob/8c13e3940b11836a4c23374d4ee607eb3424e634/nuxeo-view-designer/src/view/polymer/layout-engine.js#L21 some task variables we scaffold for each workflow layout, that are not taking into account the date capabilities and the scaffold engine we provide for regular schemas.

We should create a "virtual" task schema holding these fields and rely on our scaffolding engine to generate the model for each field.

To validate this:

• create a workflow with a task layout in Modeler (can be a copy of the default Parallel workflow)
• configure the layouts in Designer
• install the studio project in a nuxeo server instance
• create a document and start the workflow
• check that the dates are well formatted (Created and Due Date) -> check attachments