Enumerate joystick buttons
Use ACCEPT key to interact with NPCs
Add joystick support for NPC interaction

To prevent some cases of input overlapping, joystick button locking
(similar to keyboard key locking) has been added to the InputManager.
Use JOY_CANCEL to close menus
Use Accept key/button to trigger nearby events

* In checkNPCInteraction(), only lock the Accept key if an NPC is found.
* Fix a crash when trying to execute NULL events.
Use Accept key/button to pick up nearby loot
Merge pull request #339 from dorkster/joystick

Usage of Accept key/button for object interaction