qt: Unify AboutDialog and HelpMessageDialog

They share so much code and functionality that they may as well
be one class.

src/qt/utilitydialog.h

@@ -12,35 +12,16 @@ class BitcoinGUI;
 class ClientModel;
 
 namespace Ui {
-    class AboutDialog;
     class HelpMessageDialog;
 }
 
-/** "About" dialog box */
-class AboutDialog : public QDialog
-{
-    Q_OBJECT
-
-public:
-    explicit AboutDialog(QWidget *parent);
-    ~AboutDialog();
-
-    void setModel(ClientModel *model);
-
-private:
-    Ui::AboutDialog *ui;
-
-private slots:
-    void on_buttonBox_accepted();
-};
-
 /** "Help message" dialog box */
 class HelpMessageDialog : public QDialog
 {
     Q_OBJECT
 
 public:
-    explicit HelpMessageDialog(QWidget *parent, bool versionOnly);
+    explicit HelpMessageDialog(QWidget *parent, bool about);
     ~HelpMessageDialog();
 
     void printToConsole();
@@ -48,6 +29,7 @@ public:
 
 private:
     Ui::HelpMessageDialog *ui;
+    QString text;
 
 private slots:
     void on_okButton_accepted();