feat: Support Roundcube 1.7 static files handling

Roundcube 1.7 forces us to go trough static.php for security. However,
this script does not exist in older Roundcube releases. Fortunately,
there's an API function available that exists in both old and new
releases and accounts for the new behaviour.

Author: GermanCoding

README.md

@@ -25,7 +25,7 @@ The plugin name to add to your config file is: `tls_icon`.
 
 ## Requirements
 
-- Roundcube `1.3.0` or newer.
+- Roundcube `1.3.0` or newer (tested up to 1.7).
 - PHP `5.4` or newer.
 
 ## Currently supported languages