- after last meeting it was decided to not show disabled accounts
in combobox_account_selector. We are now using AvailableAccountModel.

- this patch fix several bugs :
  * display the right information on ringwelcomeview.
  * get_active_ring_account() returns the right selected account.
  * bind the selection from combobox_account_selector to
    AvailableAccountModel.
  * use currentChanged signal from selectionModel() to refresh
    treeview_conversations.
  * select the good account in AvailableAccountModel during an
    incoming call.

Change-Id: I652da29da57ab5d6d4f1abe26dd3c2cbf807396c