Add theme version checking

Add a themeversion value to the ui.xml details section. Check this
value against a define during early theme loading. If we are
loading a custom theme, then reject the custom theme if the
version does not match (or not defined) and load the stock theme.
If it is the stock theme, show a warning but continue loading.

Also fix an issue with changing overlays. Not sure how it ever
worked before, but it works now.

TLDR: Use the stock theme if the custom theme version does not
match.

Change-Id: I0277cbe035cbec7e16b6327500762dadf9c1c5c8
diff --git a/gui/theme/portrait_hdpi/ui.xml b/gui/theme/portrait_hdpi/ui.xml
index cb92b9f..917e93b 100644
--- a/gui/theme/portrait_hdpi/ui.xml
+++ b/gui/theme/portrait_hdpi/ui.xml
@@ -6,6 +6,7 @@
 		<title>Backup Naowz</title>
 		<description>Default basic theme</description>
 		<preview>preview.jpg</preview>
+		<themeversion>1</themeversion>
 	</details>
 
 	<include>