[Example App] - Add a global log control panel

[matthew-carroll]

We have a lot of loggers, and logging also has a number of verbosity levels. Typically, we activate a desired combination in main.dart for the Example app. But it can be annoying to adjust that source code list and run a hot restart every time we want to adjust the log output.

Create a hidden, but globally accessible panel, or popover, or other reasonable control that we can use to adjust logging output at runtime.