[Dev][Program Plugin] Optimize program indexing performance

[VictoriousRaptor]

This is a optimization of #1479. PATH can be a subfolder of custom program sources. It makes unnecessary I/O when enabled PATH indexing because indexing for custom sources is recursive. And a custom source can be a subfolder of another.

And let file change watcher only watch files that can be indexed (exe, `lnk', ...).

Changes

1. Refactored some code for readability.
2. Detect if PATH is a subfolder of a custom source.
3. Detect if a custom source is a subfolder of another.
4. Only watch specific file extensions.
5. Fix an issue where trying to open a .lnk files parent folder, it acutally opens its target's folder.

Tests

• Programs in PATH can be indexed correctly.
• Programs in custom sources can be indexed correctly.
• Unnecessary IO is avoided.
• Creating/Deleting a file with specifix extension triggers a reindex.

[VictoriousRaptor]

Don't want really parse the URL. So try to tell users how to avoid situaions like http actually matches a URL starts with https.

[taooceros]

why not parse it? Uri.TryParse is not expensive?

[VictoriousRaptor]

Because for steam games their URL scheme is steam://app/ rather than steam:// so parsing the URL for protocol is not enough.