Only clear cache if neccessary

[JoshuaBehrens]

As pointed out by @shyim in Shopware Gitter this plugin clears the backend cache far too often. This is a solution to ensure the captcha javascript to be in the template. If the administration frontend does not contain the captcha but the login validation expects it you are unable to login anymore.

[shyim]

Whats about setting in the required action forceCompile in Smarty to ignore the smarty cache