Debugger: Flesh out network config view.

NetworkConnectionConfigView:
- Port input field is now prepopulated with the default remote debug port
number.
- Changes to either field now trigger listener notifications.

Debugger: Add connection config handler framework.

ConnectionConfigHandler:
- Abstract base class for the different types of connection that allows one to
retrieve an appropriate configuration view based on the target host interface
type. This will allow the configuration window to switch dynamically between
network, USB, etc. without having to know the details of any of those.
Initially only a network subclass has been implemented though.

ConnectionConfigHandlerRoster:
- Keeps track of the list of available config handlers, and handles mapping a
request for a given interface info to the appropriate type of handler.

ConnectionConfigView:
- Abstract base class for the actual configuration views returned by the config
handlers. This exposes a listener interface via which the view can notify
an interested party that the configuration has been changed. Correspondingly,
the configuration window will use this to determine if the configuration is
complete enough to allow a connection attempt.