Migrate storage system to some sort of database #8
Description
(this issue is probably more effort than a beginner would be interested in spending)
Currently ActivityRecommender uses TextConverter.cs to convert from in-memory objects to text (in xml format) and back, and that text is saved into files. That means that it's difficult to delete data, even renaming an activity. That also means that some information gets duplicated, such as activity names and closing xml blocks.
It could be interesting to migrate ActivityRecommender's storage system to some sort of database, to reduce space usage and more easily allow the user to more easily make more types of changes to their data