KQuickStyleItem: Minor optimizations
Replace indirect updateItem() with direct polish() calls
The updateItem method is just a wrapper for polish() exposed to runtime meta-system by being a public slot. There is no need for it in regular C++.
Use QStringView for more efficient string processing
Hopefully someday QStringView would also support in-place slice() modifier, and built-in toLower() without temporary QString allocation.
Factor out padding calculation logic
Downgrade initStyleOption to non-virtual
The class is private, and there are no subclasses, so this method should use static dispatching for speed.