Add AprsIsConnection.State and State Change Events

[CBielstein]

Description

This switches from a simple "logged in" Boolean flag to tracking state across the full life of the AprsIsConnection object. It also adds an event which can be subscribed to for notifications of when that status changes.

This is especially useful for clients who may wish to know when a connection has been achieved, a login complete, or some error has lead to a disconnect and a new connection is required.

Changes

• Remove AprsIsConnection.LoggedIn in favor of AprsIsConnection.State
• Introduce ConnectionState enum for various states an APRS-IS connection can hold
• Updates and adds tests for validation

Validation

• Updated tests away from AprsIsConnection.LoggedIn to AprsIsConnection.State
• Added new tests for disconnected events
• All tests passing

[CBielstein]

Note to @eddywine for review: Since I add a layer of indentation for the try-finally in AprsIsConnection AND change a few lines, the diff is all weird there. It may be easier to disable whitespace in the diff. That setting is under the gear in the files tab: