Make automation expressions use common variable names and API

Purpose of this issue is not to refactor automation expression and MVEL usage, but just add consistent behaviours with JSF EL so that users are less confused.

Examples:

• add an alias for the "Document" variable to "currentDocument"
• same for "CurrentUser" vs "currentUser" in Nuxeo
• add a getPropertyValue(xpath) API on document wrapper (vs getProperty, that does not have the same behaviour on the DocumentModel interface)

Maybe more to add