Tag Browser Enhancements

[IonoclastBrigham]

Fixes #68, plus the following enhancements to the tag browser:

• Hides the thumb panel when switching to a new tag
• Adds image count to "Image List" label
• Tag search/filtering
  • Filters tag list in real time as the user types
  • Defaults to literal substring match
  • Checkbox for regex support

Known Issues

The image count label isn't displayed when there's more than 100 images associated with a tag. Looks like this might be a rendering bug in racket/gui?

[IonoclastBrigham]

Updated.