Change the way accounts connect while keeping backwards compatibility

This adds a new method to `PurpleProtocol` named `create_connection` which allows
a protocol to return a subclassed `PurpleConnection` so it can avoid using
`purple_connection_[gs]et_protocol_data`. The default implementation for this
method, just creates a `PurpleConnection` with the given protocol, account,
and password.

This also adds two new methods to `PurpleConnection`; `connect` and
`disconnect`. The default implementation for these two just call
`purple_protocol_login` and `purple_protocol_close` respectively.

The long term intent of this, is to have all protocol plugins subclass
`PurpleConnection` and then remove the `login` and `close` methods from
`PurpleProtocol` and also remove `purple_connection_[gs]et_protocol_data`.

Testing Done:
Connected the demo account and used the account actions to test reconnecting.
Connected an XMPP account after running and at startup.
Verified that the infinite loop bug on error due to connection failure is now fixed as well.

Reviewed at https://reviews.imfreedom.org/r/1967/