Use Constants instead of #define

[justinjdickow]

The parameter key and value constants defined in SDLNames.h should use extern instead of define for safe compile time type checking and to avoid collisions.

Apple's Recommendation suggests

Define constants for strings used for such purposes as notification names and dictionary keys. By using string constants, you are ensuring that the compiler verifies the proper value is specified (that is, it performs spell checking).

[joeljfischer]

Along with #7 I think this is part of a general overhaul of constants that may also include SDLEnums to be considered for making the library more internally consistent and consistent with platform best practices.

[joeljfischer]

Additionally, these defines cause greater memory use because a new string is instantiated every time the macro is used; with constants only one string would be used.