Thumbnail appearance cosmetic update

[cicciobello]

Settings -> Browser - Thumbnail -> Appearance

I think it would be useful to have these options:
- Allows the file name to be bold (and/or change its size) in the thumbnail view
- Thumbnail border corner: round
- Ability to choose the current Thumbnail background color

I see that User0 is making substantial contributions regarding the interface. Good job!ls, including those of the Tag/Rating/Color label.