Update logging for enabled devices

The previous implementation of RuntimeDeviceTracker occasionally outputted a log at level Info about devices being enabled or disabled. The problem was that the information given was inconsistent (so it would sometimes announce one change but not announce a different corrective change). This could cause weird confusions. For example, when you used a ColorTable, it would use a ScopedRuntimeDeviceTracker to temporarily force the device to Serial. The log will just tell you that the device was forced to Serial but never tell you that the devices where restored to include actual parallel devices.

This change helps correct these with the following changes:

• Added a new log level, DevicesEnabled, that is a higher level than Info. All logging from RuntimeDeviceTracker goes to this log level.
• Change the logging output of RuntimeDeviceTracker to output a list of currently enabled devices whenever a change happens. That way you don't have to guess what happend for each change.
• Change ScopedRuntimeDeviceTracker to log whenever the scope is entered or left.