Consider improvements for interaction between UI code and file operation code

[bradyt]

I think I am new to these sorts of issues. Maybe the code for the submit button, and even refresh, etc, should be decoupled. I think they're a bit tangled at the moment. I am considering it may be cognitively easier to represent the file operations with Dart's Sink/Stream objects. This way the various screens can listen for updates. For example, an indicator for, "writing to file", could be represented independently of page navigation, and considering where to add it in the UI code may become easier with the Sink/Stream model, with subscriptions, etc.