VstEffect saves and uses native path in <key <attribute type="file" value="[path]" /> /> instead of agnostic path.

### System Information

Windows as cause, all other platforms affected

### LMMS Version(s)

Since commit e30e558 / first implementation

### Most Recent Working Version

_No response_

### Bug Summary

When adding a VstEffect, a native path is used to store the information in

All currently supported platforms except Windows use / as the path separator.
Windows uses \.

Windows does support / in most operations, however.
Linux platforms do not support \.

When you load a _project that was created on Windows that contains a VstEffect_ on Linux, the VstEffect will not be found due to wrong paths.
This is not the case when loading Linux projects on Windows.

NOTE: The section is managed differently and thus is saved correctly when modified internally. It is, as far as I can see, only used for the name displayed in the window, not for the actual file being loaded.

### Expected Behaviour

Should be saved with forward slashes for cross-platform compatibility.

### Steps To Reproduce

1. Create project on Windows.
2. Add VstEffect on Windows.
3. Try to load said project on Linux.

### Logs

### Screenshots / Minimum Reproducible Project

See wrong paths from a Windows project.

### 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 tracing how VstEffect serializes its plugin path into the project XML, especially the entry, and compare that with the separately handled path. Reproduce the Windows-to-Linux loading case and verify that saved VstEffect paths use forward slashes and load correctly across platforms.

Written by the indexing model from the issue text.