tidy code a bit

- curly brace single line ifs consistently
- default trivial ctors and dtors
- prefer using over typedef
- don't declare multiple vars on the same line
- don't risk detaching containers
- be const when possible
- use ctor init list when possible
- use pragma once
- obey the rule of 5
- use explicit ctors when necessary
- use override where possible

this are partly cpp core guidelines improvements, partly general purpose
clang-tidy improvements, and also to a degree aligning better with how
we write modern code in KDE