Rewrite toInt() to fail on invalid input

[ann0see]

toInt() has a variant that sets a success indicator. That's should always be used when validating external input (even stored in a settings ini file, really).

Originally posted by @pljones in #3772 (comment)

[dingodoppelt]

I actually couldn't find anything in the Qt docs. Since we can pass the defaultValue to toInt() can we pass a boolean as well?

[ann0see]

I'd be worried to pass a bool as this could just be converted to a 0/1 automatically.

[pljones]

Hm, it might be Qt6...
https://doc.qt.io/qt-6/qintvalidator.html and https://doc.qt.io/qt-6/qvalidator.html