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/landscape_mdpi/ui.xml b/gui/theme/landscape_mdpi/ui.xml
index e29aa6b..9ab8863 100755
--- a/gui/theme/landscape_mdpi/ui.xml
+++ b/gui/theme/landscape_mdpi/ui.xml
@@ -6,7 +6,7 @@
 		<title>Backup Naowz</title>
 		<description>Default basic theme</description>
 		<preview>preview.png</preview>
-		<themeversion>4</themeversion>
+		<themeversion>{themeversion}</themeversion>
 	</details>
 
 	<include>