Use @enum for GLFW constants

[jayschwa]

ErrorCode is already an enum. It can be rolled out to more of the constants.

Candidates:

• Key
• Button actions (e.g. press, release)
• Modifier keys
• Cursors
• Window hints
• etc

The only downside is that exceptions might occur if Julia gets a value that isn't defined in the enum. This probably won't occur under normal circumstances since this package also controls which version of the GLFW library gets used. However, it might be good to write a little script that compares our enums to what is in glfw3.h.