Improve WWSK Explorer layer manager

[emxsys]

Improve the UI and functionality of the Explorer's layer manager.

• [x] Add a move-to-top button and move-to-bottom button
• [ ] ~Stack the move-up and move-down buttons one above the other~
• [ ] ~Stack the move-to-top and move-to-bottom buttons~
• [ ] ~Stack the settings and goto-layer buttons~
• [x] Move the navigation buttons to the layer settings dialog
• [x] Implement dragging in the layer manager.

[emxsys]

Fixed in the 0.5.1 hotfix.