Avoid default UI in FileDialog

[Poobslag]

We would like to use the FileDialog for importing/exporting creatures. This should be skinned to match the rest of our UI.

• [ ] tree
  • [ ] colors
  • [ ] constants
  • [ ] fonts
  • [ ] icons
  • [ ] styles
• [ ] file dialog
  • [ ] colors
  • [ ] icons
• [ ] focus matches other UI (highlighted?)

[Poobslag]

This may also be fixed by 2537.