[3.5][Feature] Added Clone operation

[tabacitu]

Fixes #28

Adds Clone operation with two actions: clone() and bulkClone(). To enable it, one only needs to:

$this->crud->allowAccess('clone');

// and maybe, if you have bulk actions:
$this->crud->addButton('bottom', 'bulk_clone', 'view', 'crud::buttons.bulk_clone', 'beginning');

IMPORTANT: The clone operation does NOT duplicate related entries. So n-n relationships will be unaffected. However, this also means that n-n relationships are ignored. So when you clone an entry, the new entry:

• will NOT have the same 1-1 relationships
• will have the same 1-n relationships
• will NOT have the same n-1 relationships
• will NOT have the same n-n relationships

This might be somewhat counterintuitive for end users - though it should make perfect sense for us developers. Hence, the Clone operation is NOT enabled by default.

Todo:

• add Clone operation to the documentation

[tabacitu]

Merged into the 3.5 release - check it out here #1647