v1.2.1

- Upgraded Abogen's interface from `PyQt5` to `PyQt6` for better compatibility and long-term support.
- Added tooltip indicators in queue manager to display book handler options (`Save chapters separately` and `Merge chapters at the end`) for queued items.
- Added `Open processed file` and `Open input file` options for items in the queue manager, instead of just `Open file` option.
- Added loading gif animation to book handler window.
- Fixed light theme slider colors in voice mixer for better visibility (for non-Windows users).
- Fixed subtitle word-count splitting logic for more accurate segmentation.
- Improvements in code and documentation.