Feature/dev 1159 improve structure events

[brandonkelly]

Description

• Adds new EVENT_BEFORE_INSERT_ELEMENT and EVENT_AFTER_INSERT_ELEMENT events to craft\services\Structures
• Adds craft\events\MoveElementEvent::$targetElementId and getTargetElement(), for getting the target element that the action is being performed in reference to, if any. (These will be null if $element is being prepended/appended to the root of the structure.)
• Adds craft\events\MoveElementEvent::$action, which will be set to craft\services\Structures::ACTION_PREPEND, ACTION_APPEND, ACTION_PLACE_BEFORE, or ACTION_PLACE_AFTER.
• craft\events\Structures::EVENT_BEFORE_MOVE_ELEMENT is now cancellable, so setting $isValid to false will prevent the element from getting moved.
• Deprecates craft\base\Element::beforeMoveInStructure() and afterMoveInStructure(), as well as the EVENT_BEFORE_MOVE_IN_STRUCTURE and EVENT_AFTER_MOVE_IN_STRUCTURE events they trigger, as they were named consistently with craft\services\Structures::EVENT_BEFORE_MOVE_ELEMENT and EVENT_AFTER_MOVE_ELEMENT, however they didn’t behave the same (they were also triggered for insert operations, whereas the service events were only for actual moves).

Related issues

• Add targetElementRecord to before/after move element in structure function and events #13409

[linear]

DEV-1159 Improve structure events