Revisit txt-previews

[nickvergessen]

.txt previews are generated with a white background, which is now displayed on white background instead of gray background compared to before
owncloud-archive/apps#1769 (comment)

Might not be an issue for normal files as they are not as empty as the one-line testing files I used

[jancborchardt]

Normally I would suggest adding a slight border around the preview, but that would make the filetype icons look bad. And honestly, this is only an issue occuring with one-line or completely empty text files.

I’ll close this hence. We can discuss it again if it comes up in production use.