Add scan for external changes to Files/Project

### Description

Currently youn can run "scan for external changes" to do a project wide scan.
That is fine, if all sources are local and not on a network folder e.g. on davfs,, which is quite slow.

It would be nice, if this function is available on a folder base in projects or files. So you can scan for changes in that folder only. This would speed up the process.

At least, if this can be added to one project and not scan all projects.

### Use case/motivation

if you have a share where files change or are added, you have to sometimes scan for external changes. This is quite slow.

### Related issues

_No response_

### Are you willing to submit a pull request?

No

### Code of Conduct

Yes

The issue does not name any files, tests, or entry points. Start by locating the existing project-wide “scan for external changes” action in NetBeans, then determine how folder- or project-scoped scanning should be exposed and verified; done means scanning only the selected folder or project without traversing unrelated projects.

Written by the indexing model from the issue text.