Code cleanup for ITMApplication

[tcobbs-bentley]

Also contains updates to other classes that are a direct result of ITMApplication changes.

Notes:

1. The ITMMessenger changes were done to avoid potential issues that previously required the suppression of LeakingThis. Changing logger to observable and updating the messenger when the logger changes maintains the old functionality.
2. The ITMNativeUI changes were also done to avoid having to suppress LeakingThis, and to avoid running into problems due to that suppression.
3. The ITMWebViewLogger change was due to fixes added to ITMApplication to detach the logger when appropriate.