Fix: Linux clipboard fallback skips xsel when xclip is not installed

[TofuGG]

What does this PR do?

On Linux, copy_to_clipboard() tried xclip and xsel as fallbacks inside a single shared try/except block. If xclip wasn't installed, the FileNotFoundError it raised escaped the loop and was caught by the outer handler — meaning xsel was never tried. Users with xsel but not xclip (common on Debian/Ubuntu) would silently fail to copy even though a working tool was present.

The fix wraps each command in its own try/except inside the loop, so a missing tool just continues to the next candidate. The macOS and Windows branches are also given their own try/except blocks for the same reason.

Three regression tests are added covering: xsel succeeding when xclip is absent, xclip succeeding when xsel is absent, and False returned cleanly when neither tool is installed.

Related issue

No related issue.

Checklist

• Tests pass (make test)
• New behaviour is covered by tests (if applicable)
• CHANGELOG.md updated under [Unreleased]

## [Unreleased]

### Fixed
- Clipboard fallback on Linux now correctly tries `xsel` when `xclip` is not
  installed. Previously both commands shared a single `try/except`, so a
  `FileNotFoundError` from `xclip` aborted the loop before `xsel` was ever
  attempted.