Rework InventoryClickEvent for 1.5 changes

Two new events have been added, InventoryCreativeEvent and InventoryDragEvent.
Additionally, a common superclass has been created, InventoryActionEvent.

InventoryActionEvent now has an enum, InventoryAction, representing the
different kinds of manipulations a player can perform in their inventory and
other containers.

Thanks to @YLivay for his InventoryPaintEvent pull request.