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.

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

[Poobslag]

This may also be fixed by #2537.