Add g_socket_connection_connect(), etc

Add g_socket_connection_connect(), etc

Previously it was more or less assumed that GSocketConnections were
always connected, although this was not enforced. Make it explicit
that they don't need to be, and add methods to connect them, and
simplify GSocketClient by using those methods.

https://bugzilla.gnome.org/show_bug.cgi?id=665805