Replace "Master Key" with "Database Credentials"

Definitions:
* Database Key - Cryptographic hash used to perform encrypt/decrypt of the database.

* Database Credentials - User facing term to refer to the collection of Password, Key File, and/or Hardware Key used to derive the Database Key.

Changes:
* Remove the term "master" and "key" from the user's lexicon and clarify  the code base based on the definitions above.
* Clean up wording in the UI to be clearer to the end user.

src/gui/databasekey/YubiKeyEditWidget.h

@@ -0,0 +1,57 @@
+/*
+ *  Copyright (C) 2018 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_YUBIKEYEDITWIDGET_H
+#define KEEPASSXC_YUBIKEYEDITWIDGET_H
+
+#include "KeyComponentWidget.h"
+#include <QPointer>
+
+namespace Ui
+{
+    class YubiKeyEditWidget;
+}
+
+class YkChallengeResponseKey;
+
+class YubiKeyEditWidget : public KeyComponentWidget
+{
+    Q_OBJECT
+
+public:
+    explicit YubiKeyEditWidget(QWidget* parent = nullptr);
+    Q_DISABLE_COPY(YubiKeyEditWidget);
+    ~YubiKeyEditWidget() override;
+
+    bool addToCompositeKey(QSharedPointer<CompositeKey> key) override;
+    bool validate(QString& errorMessage) const override;
+
+protected:
+    QWidget* componentEditWidget() override;
+    void initComponentEditWidget(QWidget* widget) override;
+
+private slots:
+    void hardwareKeyResponse(bool found);
+    void pollYubikey();
+
+private:
+    const QScopedPointer<Ui::YubiKeyEditWidget> m_compUi;
+    QPointer<QWidget> m_compEditWidget;
+    bool m_isDetected = false;
+};
+
+#endif // KEEPASSXC_YUBIKEYEDITWIDGET_H