Recursive Search in LMMS

### Enhancement Summary


Kdenlive (a video editor) has a feature that allows you to locate all the files used in your project. With this button, you can select the folders where those files are located. The program will automatically detect the files and reload them into the project if you want to open your project on another computer, avoiding having to manually locate each file used in the project.

### Implementation Details / Mockup

If this feature is included in LMMS, you will be able to locate all the files used in your project, such as audio samples, plugins, presets, soundfonts, etc. This makes LMMS projects more portable. If you want to copy your project to another computer, you just have to put all the files used along with the project file in the same folder. The recursive search feature will take care of locating each file and reloading it into the project.

### Please search the issue tracker for existing feature requests before submitting your own.

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

The issue does not name any files, tests, or entry points; begin by locating LMMS's project file and referenced-asset loading code. Done means recursively finding audio samples, plugins, presets, and soundfonts in selected folders, reloading them into the project, and supporting project portability as described.

Written by the indexing model from the issue text.