#17 ER: Allow disconnected sessions to be reconnected

[Christoph Daniel Schulze]

When an IRC connection has successfully been
established, disconnecting it closes its tabs in the
main window. If a connection couldn't be established,
its tab cannot be closed. Neither can another try at
connecting to the server be made without opening a new
tab. Such, the following is needed:

* The Disconnect action needs to be able to close the
tabs of disconnected sessions.

* A Reconnect action is needed which tries to reconnect
a disconnected session.