WIP: Add an experimental watcher for testing notification handling

Right now this module fails to do any book keeping to manage the
subscription.
Only include 'Expires' header in a subscribe request if provided

The use of nksip_lib:get_integer/2 means that when undefined, it
defaults to zero.  This removes the possibility of relying on the
default expiration for the package.
When handling subscription dialogs, update the associated response

This allows us to lean on the existing mechanism for updating the
remote_target; necessary for refreshing subscriptions within a
dialog.

It might be advisable to revisit this to make sure that there are no
INVITE-specific mechanisms at work here.
Provide the ability to reSUBSCRIBE within an existing dialog

This requires capturing the previous request in the dialog in order to
obtain the 'Event' package automatically.  The mechanism involved is
similar to the remote_target, which requires the previous response.

This ensures we don't have to manually specify the required event
package.

A revisit of this functionality might be necessary to ensure we're not
leaning on any INVITE-specific functionality.
WIP: Expand dialog handling for SUBSCRIBE requests
WIP: Subscription refreshing for SipApp