[INTERNAL][CORE] Amends XMPPAccountStore API

Introduces get/set defaultAccount methods which should be a replacement
for set/get activeAccount in the future. 

They allow null values making the call to isEmpty obsolete. And now it
is also possible to completely remove all account data.

Add getAccount methods as there was no way to retrieve specific account
unless you called getAllAccounts and perform the search by yourself.

Author: srossbach

core/src/saros/account/XMPPAccount.java

@@ -136,8 +136,7 @@ public String toString() {
         + ", TLS: "
         + useTLS
         + ", SASL: "
-        + useSASL
-        + " : ";
+        + useSASL;
   }
 
   @Override