Changes to internal data handling and other important fixes

This is a significant change to how the app works internally. It fixes some issues (hopefully without introducing new ones 😅) and simplifies things a bit to (hopefully) make it easier to implement some of the other planned features.

fixes #14 (closed)

Note: I don't expect many people to actually review all of the changes (though if you want to, please don't let me stop you 😃). This MR is here so that others can help test.