refactor(preferences): conditionally activate the Sieve script

All the user defaults are now editable through the Preferences module,
even if an external Sieve script is enabled. However, the user can
disable the external Sieve script and force the activation of the
"sogo" Sieve script.

SoObjects/SOGo/SOGoSieveManager.h

@@ -50,10 +50,13 @@
                         withUsername: (NSString *) theUsername
                          andPassword: (NSString *) thePassword;
 
+- (BOOL) hasActiveExternalSieveScripts: (NGSieveClient *) client;
+
 - (BOOL) updateFiltersForAccount: (SOGoMailAccount *) theAccount;
 - (BOOL) updateFiltersForAccount: (SOGoMailAccount *) theAccount
                     withUsername: (NSString *) theUsername
-                     andPassword: (NSString *) thePassword;
+                     andPassword: (NSString *) thePassword
+                 forceActivation: (BOOL) forceActivation;
 
 @end