Improve "Apply" button functionality by adding background processing indication

[BeatEhrmann]

When clicking the "Apply" button for polynomial and expansion calculation, it would be beneficial to open a thread that processes the task in the background. During this time, an indication should be shown to inform the user that the calculation is still being processed.

Suggested Improvements:

1. When the "Apply" button is clicked, start a background thread for processing the task.
2. Add a visual indication (e.g., a loading spinner or progress bar) to notify the user that the process is ongoing.
3. Ensure that the indication is cleared once the calculation is completed and the results are ready.

This enhancement would improve user experience by making the process more transparent and responsive.