File Extension management interface (#659)

[NEW] Add new interface and functionality to manage database file
extension. (Implements feature request #659)

src/PreferencesDialog.h

@@ -36,9 +36,13 @@ private slots:
     void chooseRemoteCloneDirectory();
     void updatePreviewFont();
 
+    void on_buttonManageFileExtension_clicked();
+
 private:
     Ui::PreferencesDialog *ui;
 
+    QStringList m_dbFileExtensions;
+
     void fillLanguageBox();
     void loadColorSetting(QFrame *frame, const QString &name);
     void setColorSetting(QFrame *frame, const QColor &color);