Add an experimental gui

- Requires 10.15 for SwiftUI & Combine frameworks

Update swift.yml

- Removed macOS-10.14 build because it is no longer supported by github actions

Updated CI config

Fix & Improve:

- Refactor, rename functions
- Add date mocking in tests
- Fine tuning

Fixes & Improvements:

- Removed flaky test (depending on sort() being stable)
- Changed report interface

Updates:

- Changed date formatting in 'log' commands
- Use proper NULLs instead of empty strings in database
- Help message
- Added Tests
- Moved configuration to the interface part

Implemented 'report' and 'log' commands

- Matrix valued SQL Queries and tests
- Stuff

Refactoring & Other

- Split up class Database
- Tests, adapted function signatures for testing
- Reduced code duplication
- Use default rating instead of computed

Installation of the launchd configuration

- 'idbcl install' creates an example launchd configuration
- Renamed variables
- Makefile: Patch executables library path

Improvements & Fixes

- Fix crash on empty track property
- Added 'emit-plist' command: Creates the (example) idbcl.plist to be installed to ~/Library/LaunchAgents/