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.

This patch introduces the usage of the new XMPPStore API.

Renaming UI buttons is not done yet as changing UI values is likely to
crash the whole STF.

This patch also fixes an issue in the getAccountsMethods so they lookup
the account correctly by ignoring the case sensitivity of the username.

Adds a FIXME to the XMPPAccount class regarding RFC 6122