In general +1 on the concept, but needs revision before it can go in.
Put it in between Zoom in and Zoom out, as it is in Okular and Konsole.
Name should be "Reset zoom level" or "Zoom to default size"
That seems like a fairly arbitrary value to reset it to. You should read the default size and zoom to that instead. After all, that's what the WhatsThis text you wrote says it will do. :)
Cool, it works and the code is looking pretty good. Now I think the new action needs to be added to the View menu too, between the zoom in and zoom out menu items. The place where you've added it in dolphinui.rc doesn't actually do that.
Sorry, I steered you wrong. The text needs to Use Title Case:
"Reset zoom level"
Ah, looks like Dolphin conditionally adds the zoom actions to the main menu in dolphinmainwindow.cpp (lines 984-988). That's where you should add the new action. Then I guess you don't need to touch the rc files.
Still not fixed ;)
Please remove the spaces before/after QStringLiteral
Unrelated change, should go to its own commit. (feel free to push it without review...)
Sorry but I don't like this solution (i.e. creating a settings tab widget inside the action handler). Please try the following instead: