✨✨ CLI Command cleanup ✨✨

This PR cleans up the `Command` classes in the CLI, introducing a
`DatabaseCommand` class for the commands operating on a database,
and a `getCommandLineParser` command to centralize the arguments
parsing and validation.

The opening of the database based on the CLI arguments and options
is now centralized in `DatabaseCommand.execute`, making it easy to
add new database opening features (like YubiKey support for the CLI).

Also a couple of bugs fixed:
  * `Create` was still using `stdout` for some error messages.
  * `Diceware` and `Generate` were not validating that the word count was an integer.
  * `Diceware` was also using `stdout` for some error messages.

src/cli/DatabaseCommand.h

@@ -0,0 +1,35 @@
+/*
+ *  Copyright (C) 2019 KeePassXC Team <team@keepassxc.org>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 2 or (at your option)
+ *  version 3 of the License.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef KEEPASSXC_DATABASECOMMAND_H
+#define KEEPASSXC_DATABASECOMMAND_H
+
+#include <QCommandLineOption>
+
+#include "Command.h"
+#include "Utils.h"
+#include "core/Database.h"
+
+class DatabaseCommand : public Command
+{
+public:
+    DatabaseCommand();
+    int execute(const QStringList& arguments) override;
+    virtual int executeWithDatabase(QSharedPointer<Database> db, QSharedPointer<QCommandLineParser> parser) = 0;
+};
+
+#endif // KEEPASSXC_DATABASECOMMAND_H