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/Render.cpp

@@ -12,13 +12,13 @@
 #include "UtilityClass.h"
 #include "Version.h"
 
-int tr;
-int tg;
-int tb;
+static int tr;
+static int tg;
+static int tb;
 
 // Macro-like inline function used in maprender()
 // Used to keep some text positions the same in Flip Mode
-int inline FLIP(int ypos)
+static int inline FLIP(int ypos)
 {
     if (graphics.flipmode)
     {
@@ -46,7 +46,7 @@ static inline void drawslowdowntext()
     }
 }
 
-void menurender()
+static void menurender()
 {
     int temp = 50;