BUG Fixes #144. Users with limited permissions can delete workflows.

- Added canCreate/canEdit/canDelete() to Definition, Action & Transition.
- canEdit+canDelete call WorkflowDefinition#canCreate().
- Made WorkflowField UI buttons disabled based on the canXX() methods.
- Removed disableButton() methods, using canXX() in template directly.
- Added missing docblocks & tests.