The global variable "Database" of class xca_db
can be used by any other class to access. No need
to provide it.
The "xca_db" class managed the "database_model" pointer.
Accessing the name of the current database has been unified.
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
