Provide function in rcube to connect to IMAP

This is useful for plugins like FreeBusy [1] that previously
connected to IMAP via plain rcube_imap::connect().

[1] https://git.kolab.org/source/freebusy/browse/master/lib/Kolab/FreeBusy/SourceIMAP.php

program/include/rcmail.php

@@ -764,13 +764,7 @@ public function login($username, $password, $host = null, $cookiecheck = false,
         $storage = $this->get_storage();
 
         // try to log in
-        if (!$storage->connect($host, $username, $password, $port, $ssl)) {
-            if ($user) {
-                $user->failed_login();
-            }
-
-            // Wait a second to slow down brute-force attacks (#1490549)
-            sleep(1);
+        if (!$this->imap_connect($storage, $host, $username, $password, $port, $ssl, $user)) {
             return false;
         }
 
@@ -782,7 +776,6 @@ public function login($username, $password, $host = null, $cookiecheck = false,
             return true;
         }
 
-
         // user already registered -> update user's record
         if (is_object($user)) {
             // update last login timestamp