Vanilla Ice Cream Update

• Bumped the target SDK from version 34 (Upside-down Cake) to 35 (Vanilla Ice Cream)
• Major refactoring of the data and service layers to make automated testing easier
• Added lots more automated tests
• The importers now stream data from input files instead of reading the whole file into memory before processing, which allows much larger files to be imported.
• Fixed a UI bug where the pop-up list of available due dates was not updated after one or more days had passed since the app started.
• Fixed a bug where rotating the screen while in the details window would cause that activity to crash and either return to the list window or become unusable. Fixed a related issue where rotating the screen while editing an item's note would cause all changes to be lost.