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: #831, #1221

src/cli/Utils.cpp

@@ -59,15 +59,15 @@ void Utils::setStdinEcho(bool enable = true)
 QString Utils::getPassword()
 {
     static QTextStream inputTextStream(stdin, QIODevice::ReadOnly);
-    static QTextStream outputTextStream(stdout, QIODevice::WriteOnly);
+    static QTextStream errorTextStream(stderr, QIODevice::WriteOnly);
 
     setStdinEcho(false);
     QString line = inputTextStream.readLine();
     setStdinEcho(true);
 
     // The new line was also not echoed, but we do want to echo it.
-    outputTextStream << "\n";
-    outputTextStream.flush();
+    errorTextStream << "\n";
+    errorTextStream.flush();
 
     return line;
 }

src/core/Database.cpp

@@ -465,8 +465,8 @@ Database* Database::unlockFromStdin(QString databaseFilename, QString keyFilenam
     QTextStream outputTextStream(stdout);
     QTextStream errorTextStream(stderr);
 
-    outputTextStream << QObject::tr("Insert password to unlock %1: ").arg(databaseFilename);
-    outputTextStream.flush();
+    errorTextStream << QObject::tr("Insert password to unlock %1: ").arg(databaseFilename);
+    errorTextStream.flush();
 
     QString line = Utils::getPassword();
     PasswordKey passwordKey;