When XCA started in 2002, there were no C++ initializers.
Drop explicit initializers from the constructors.
- Fix indentations of section declarators.
- Replace NULL by nullptr when feasible.
- Sort private section: properties first, then methods.
When using a key with a hash algorithm, check whether
they can be used together. This should have been happened in the GUI.
In case of an error use the largest algorithm
