PreviewSettings: add a tooltip showing the filename of plugins

So users can identify them and distinguish them when two plugins provide the same mimetype support or help figure out devs where the plugins are loaded from.

Best with frameworks/kio!2027 (merged) that adds the fileName for the standardthumbnailer case.

This is to assist users https://discuss.kde.org/t/duplicated-entries-in-dolphin-preview-setting/40563/

second commit applies the same logic to the contextual menu settings.