theme: move TW_THEME_VERSION to variables.h

Automatically add the current theme version to the
splash and ui xmls during the build

Change-Id: I52f8cb41fabe34a8849ecc54ece3c0fb645dcd74
(cherry picked from commit a1317fc7bbba9b26bb65d207bd48f7f79b7e98fd)
diff --git a/gui/pages.cpp b/gui/pages.cpp
index bc4a936..8b9ce46 100755
--- a/gui/pages.cpp
+++ b/gui/pages.cpp
@@ -53,9 +53,7 @@
 #include "objects.hpp"
 #include "blanktimer.hpp"
 
-// version 2 requires theme to handle power button as action togglebacklight
-// version 4 adds fastbootd support
-#define TW_THEME_VERSION 4
+#include "../variables.h"
 
 #define TW_THEME_VER_ERR -2