Make MXMIMAPAccount immutable

It's a good habit to make data structures immutable by default unless
they have to be otherwise. Make MXMIMAPAccount immutable by making the
properties read-only and initializing them from the constructor. This
also removes some of the complexity from the application delegate.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Add server port and TLS usage to account config

We should be able to specify the port number on the server and if TLS
should be used when communicating with it. Use TLS by default and if
it's enabled set the default port to 993, otherwise 143.

This is probably not going to be enough in the long run. We might for
example want to specify the specific authentication method that we want
to use or more detailed TLS configuration such as a client certificate.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>