SSH port forwarding is now configurable in mid-session. After doing

SSH port forwarding is now configurable in mid-session. After doing
Change Settings, the port forwarding setup function is run again,
and tags all existing port forwardings as `do not keep'. Then it
iterates through the config in the normal way; when it encounters a
port forwarding which is already in the tree, it tags it `keep'
rather than setting it up from scratch. Finally, it goes through the
tree and removes any that haven't been labelled `keep'. Hence,
editing the list of forwardings in Change Settings has the effect of
cancelling any forwardings you remove, and adding any new ones.

The SSH panel now appears in the reconfig box, and is empty apart
from a message explaining that it has to be there for subpanels of
it to exist. Better wording for this message would be welcome.

[originally from svn r5030]