Add "Open Custom Layouts Folder" button to Layouts preferences

[rgeraskin]

Add "Open Custom Layouts Folder" button to Layouts preferences

Closes #1434

What's Changed

Added a new button in the Layouts preferences tab that allows users to quickly open the custom layouts directory in Finder.

Why This Feature?

• Better User Experience: Users no longer need to manually navigate to ~/Library/Application Support/Amethyst/Layouts/
• Improved Discoverability: Makes it easier for users to find where to place custom layout JavaScript files
• Consistency: Follows macOS app patterns of providing direct access to user data directories

How It Works

• Button is positioned to the left of the existing "Relaunch Amethyst" button
• Clicking "Open Custom Layouts Folder" opens the layouts directory in Finder
• If the directory doesn't exist, it's automatically created (via existing layoutsDirectory() method)
• Graceful error handling with logging if directory access fails

Implementation Details

• UI Changes: Added button with proper Auto Layout constraints in LayoutsPreferencesViewController.xib
• Controller Changes: Added @IBOutlet and @IBAction in LayoutsPreferencesViewController.swift
• Leverages Existing API: Uses the existing FileManager.layoutsDirectory() extension method
• Native macOS Integration: Uses NSWorkspace.shared.open() to open directory in default file manager

Files Modified

• Amethyst/Preferences/LayoutsPreferencesViewController.xib - Added button UI and constraints
• Amethyst/Preferences/LayoutsPreferencesViewController.swift - Added outlet and action method

User Benefit

This enhancement reduces friction for users who want to create or manage custom layouts, making the feature more accessible and user-friendly.