[Feature Request] Allow users to create custom Categories

[MarquisLP]

This is the most popular user request by a large margin, and indeed it's something that should have been included from the get-go.

Unfortunately, the way the app is structured right now makes it very difficult to implement this feature without significant changes to the whole project. Aside from the changes in file operations and business logic, the UI will also need to be reworked (the 5 category buttons on the bottom already take up all of the available space).

[tostc]

For the gui the bottom bar could be scrollable, or all categories must structured in a list either in a side menu or as fragment or activity.