Consider all thumbnailers for the information panel and tooltips

Summary:
The text thumbnail is really useful in tooltips, but disabled by default in
KIO. Having just a larger version of the placeholder icon from the theme
doesn't make a lot of sense, so try the best to get a proper preview.
Same applies to the information panel, which would otherwise just
show the same icon.

Test Plan:
Got textfile previews in tooltips and in the information panel even if
disabled.

Reviewers: Dolphin, elvisangelaccio, broulik, markg, ngraham

Reviewed By: Dolphin, broulik, ngraham

Subscribers: abetts, markg, kfm-devel, ngraham

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D15138