What's the reason for loader becoming public? If that's mostly out of convenience (it looks like it is), I'd advise not turning it public, Settings and ConfigView can use it internally but let's not increase the API surface even more with Loader.

Obviously will reappear here ;-)

This is an odd bit of logic, isn't it?

Ditto

Please remove that include (it'll become more obvious why you'll be able to do it with one of the comments below) ;-)

Will need a ctor (see other comments) in the form of explicit Settings(QObject *parent = nullptr);

This is likely useless now, since we inherit from QObject and QObject is non-copyable

Ditto

Move all the implementations of those static methods to the cpp file.

Time to move those to the SettingsPrivate class.

Might end up disappearing completely, unsure but to be evaluated

And that negation again? We're negating twice now...

And again...

I wonder what's happening there, it's highly confusing.

I'm kind of surprised at this extra widget, can't we get rid of it?

Please move this line just after the #include <QWidget>

You might want to declare a bunch of Q_PROPERTY there

clients should be a const reference (although I suspect it'll disappear)

parent should be the last parameter and default to nullptr

override

All the setters there would make for good public slots

Space before & not after

Are they directly called by a subclass outside the library? If not move them to the dpointer and use Q_PRIVATE_SLOT

Move to the d pointer and use Q_PRIVATE_SLOT for those

Should be in the d pointer

Please rename it to "skip uppercase"...

Otherwise it does the opposite of its name, it's just confusing

Likewise should be renamed to "skip"

Break the line before { please

Rename the parameter to skip as well please

I'd expect a negation here... if by default skip is false, check in settingsimpl should be true (which was the old default).