Qt Designer: Fix clang-tidy warnings about Qt usage

- Use multi-arg form of QString::arg()
- Fix static method calls
- Prefer QFileInfo::exists()
- Use isEmpty() for containers instead of checking size()
- Use QOverload

Change-Id: I31e92f77d0fa377932c1369afe2503abc8ecceac
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>