Clean up unnecessary exports and add static keywords

This patch cleans up unnecessary exports from header files (there were
only a few), as well as adds the static keyword to all symbols that
aren't exported and are specific to a file. This helps the linker out in
not doing any unnecessary work, speeding it up and avoiding silent
symbol conflicts (otherwise two symbols with the same name (and
type/signature in C++) would quietly resolve as okay by the linker).

desktop_version/src/Game.cpp

@@ -26,7 +26,7 @@
 #define strcasecmp stricmp
 #endif
 
-bool GetButtonFromString(const char *pText, SDL_GameControllerButton *button)
+static bool GetButtonFromString(const char *pText, SDL_GameControllerButton *button)
 {
     if (*pText == '0' ||
         *pText == 'a' ||