Release Candidate for Back in Time 1.5.3

Please allow me to announce a new Release Candidate in preparation for the Back In Time release version 1.5.3. Please report back if you use it, even if no issues are found. Details: https://github.com/bit-team/backintime/releases/tag/v1.5.3-r...

Changes: - Extensive code refactoring. - Added support for FCron. - Improved text labels in the graphical interface. - User manual updated. - Serbian language now available in both Latin and Cyrillic scripts. - New language: Interlingua (Occidental). - Minimum supported Python version raised to 3.9. - Removed dependency on libnotify-bin; now using DBus for notifications.

We encourage everyone to test this Release Candidate, as it helps ensure a higher-quality final release. Without testing and feedback, creating an Release Candidate would be pointless. Your input is essential to improving the project!

Thank you very much for support!