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/theme/watch_mdpi/splash.xml b/gui/theme/watch_mdpi/splash.xml
index 62e8dc3..2ce71b3 100644
--- a/gui/theme/watch_mdpi/splash.xml
+++ b/gui/theme/watch_mdpi/splash.xml
@@ -5,7 +5,7 @@
 		<author>TeamWin</author>
 		<title>TWRP</title>
 		<description>splash screen</description>
-		<themeversion>3</themeversion>
+		<themeversion>{themeversion}</themeversion>
 	</details>
 
 	<resources>