Create better methods for querying plugin states
Transport plugins can be in the following states:
- No plugin created
- Failed to start
- Started but disabled by settings
- Started and enabled, but unable to connect
- Started, enabled, and able to connect
The Plugin#isRunning() method returns true if the plugin is started, enabled, and able to connect, and false for all other states. It would be useful for the UI and core to be able to get more information about plugin states.
See discussion on #185 (closed).