Write CLI password prompt to stderr

Writing the prompt to stderr greatly helps with the use of keepassxc-cli
in automated scripts or as a password provider for programs like mutt or
borg.

This is done in accordance with POSIX that specifies that diagnostic
messages be sent to stderr.

This commit should be a first step in solving the following
issues: keepassxreboot#831, keepassxreboot#1221

Author: lhark

src/core/Database.cpp

@@ -507,8 +507,8 @@ Database* Database::unlockFromStdin(QString databaseFilename, QString keyFilenam
     QTextStream out(outputDescriptor);
     QTextStream err(errorDescriptor);
 
-    out << QObject::tr("Insert password to unlock %1: ").arg(databaseFilename);
-    out.flush();
+    err << QObject::tr("Insert password to unlock %1: ").arg(databaseFilename);
+    err.flush();
 
     QString line = Utils::getPassword();
     auto passwordKey = QSharedPointer<PasswordKey>::create();