Add the themes configuration entry point

Change-Id: I0eb669956d9e89bb92cd43487b14b4246454a400

Conflicts:
	res/values/strings.xml

 Author:    Ricardo Cerqueira <cyanogenmod@cerqueira.org>
 On branch jb422
 Changes to be committed:
	new file:   res/drawable-hdpi/ic_settings_themes.png
	new file:   res/drawable-mdpi/ic_settings_themes.png
	new file:   res/drawable-xhdpi/ic_settings_themes.png
	modified:   res/values/strings.xml
	modified:   res/xml/settings_headers.xml