Change GUI Slot Update to before event, and Allow additions to T1 Chests

Previously, the getSelectedDimension method in the GCCoreGuiChoosePlanet
instance would return the old slot, not the newly selected one. This
causes the change to happen before the event is sent out, and event
listeners can get the newly selected slot.

Second, a small api change that allows external addons/mods to add items to spawn in moon dungeon chests.