Moved WindowEvents to API package.

Created AbstractWindow.  VanillaRenderMaterials is also now in the API package as it's a static class and seems to be universally necessary.

Updated Docs for InventoryCanSetevent.  removed PlayerSlotChangeEvent, it was unnecessary to begin with.

Signed-off-by: Nick Minkler <sleaker@gmail.com>
Merge pull request #553 from Sleaker/master

Move WindowEvents to API packages