Validate form fields in a Workflow when saving

When building the Workflow feature, a schema is added, grouping the global variables that are edited within a node. But, if a widget on a node's form is bound to an non-existing field, a NPE is thrown and the project is not build.

To avoid this, we should implement a validation of the fields on each form of a workflow when saving.

Steps to reproduce:

• Create a workflow variable
• Use this workflow variable in a node's form tab
• Save the workflow
• Delete the workflow variable
• Save the workflow again