GafferUI : Fix local documentation links on Windows

[pascalandr]

Fixes local documentation links such as Help > User Guide and Help > Node Reference on Windows.

showURL() currently strips the file:// prefix and passes the resulting drive-letter path to QUrl. Qt interprets the drive letter as the URL scheme (c:), so QDesktopServices does not open the local HTML file. This change uses QUrl.fromLocalFile() for that Windows-specific branch.

• Preserve the existing removal of URL fragments on Windows.
• Preserve existing web URL handling.
• Add coverage for HTTPS URLs and Windows local file URLs containing spaces and fragments.

Related issues

• None.

Dependencies

• None.

Breaking changes

• None.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. No documentation change is required.
• I have tested my changes and added a regression test.
• My code follows the project's prevailing style and conventions.

Testing

Validated with the Qt bindings shipped in Gaffer 1.7.1.0 on Windows. The resulting URL uses the file scheme, reports isLocalFile() == True, preserves paths containing spaces, and strips the unsupported fragment. HTTPS URL handling is unchanged.

The full repository test suite was not run because this checkout has no compiled build; the targeted behavior and Python syntax were tested against the installed Gaffer 1.7.1.0 runtime.