Avoid impossible delete actions

[Guite]

Database constraints ensure integrity of stored data. Because of that it is not possible to delete a parent object if it has still children. Therefore according delete actions will fail.

Possible solution:

• Make delete button inactive
• Tooltip showing a message with the reason (existing children)
• More enhanced options for later (see Provide enhanced options on deletion of parent objects #14)

Notes:

• This function does not make sense in all use cases. It must consider the different cascade types.
• Code template available in a project where this has been implemented as a isDelete() entity method. Generator integration should wait until Migrate to Doctrine 2.1 #21 is done though.

[Guite]

This will be done for ZK 1.5+ only, because the new workflow system makes this much simpler to implement.