More work on InventoryEvents.

I've removed InventoryCreativeEvent#getItem and
InventoryCreativeEvent#setItem and simply mapped those to the cursor.
Based upon how InventoryClickEvent handles cursor(the cursor being
modified before the changes are applied to the inventory), this behavior
is therefore consistent between InventoryClickEvent and
InventoryCreativeEvent.  To further improve the consistency and remove
any potential confusion with cursors, all InventoryCreativeEvents use
InventoryAction.PLACE_ALL.

Additionally, methods in InventoryClickEvent were moved to ClickType if
they weren't required to maintain compatability.

Javadocs have been improved and correct, and formatted issues have been
addressed.

InventoryClickEvent#setCursor was deprecated with an explanation of why
provided in the javadocs.