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.
New classes:
- database_model: Manage the database tables (keys, req, cert, crl, templ)
extracted from widgets/MW_database.cpp
- arguments: parse commandline, dynamically create help text
