Make AccountModel loading asynchronous.
AccountManager starts up in 3 steps, first waiting for credentials and
then for Servent to be ready. During this time an AccountModel could be
instantiated in the GUI thread, which needs AccountManager, and if the
latter is not ready at that time, the former cannot be populated with
sane data.
This commit splits the AccountModel ctor so that it waits for
AccountManager to be ready before hooking up to its signals and
performing the initial model reset.