Consider declare & define macro pairs for SDL definitions

[TzuChieh]

Currently, SDL definitions are done via a series of macros, e.g., PH_DEFINE_SDL_CLASS(). This can slow down the compiler quite a lot since they are template heavy. Consider to provide a declaration variant for such macros, so definitions will not need to be compiled in other TUs.

This issue should be analyzed when the compile time exceeds 15 mins (longer than my most short breaks, hence reducing productivity).