FileDialog::FileDialog(...) resets (and overwrites) the Sidebar URLs in LMMS and other QT apps

### System Information

Archlinux

### LMMS Version(s)

nightly

### Most Recent Working Version

_No response_

### Bug Summary

When the file picker is launched in LMMS (via exporting, opening or other operations), previous bookmarks from LMMS and other Qt applications are overwritten and lost.

### Expected Behaviour

Previous bookmarks/configuration of the URL Sidebar remains.

### Steps To Reproduce

1. In other QT apps (e.g Krita), open the file dialog and add any bookmark.
2. Open the dialog again in that app (e.g Krita) to verify that the bookmark is there.
3. Open LMMS and activate the file dialog.

### Logs

Click to expand

### Screenshots / Minimum Reproducible Project

https://github.com/user-attachments/assets/c12913d3-45aa-4ba0-b402-215fff58a790

### Please search the issue tracker for existing bug reports before submitting your own.

- [X] I have searched all existing issues and confirmed that this is not a duplicate.

Start by reproducing the file dialog through exporting, opening, or another LMMS operation, then compare the URL Sidebar behavior with Krita or another Qt application. Done means bookmarks and other sidebar configuration created in LMMS or another Qt app remain intact after launching LMMS's file picker.

Written by the indexing model from the issue text.