Add -version option to get just the version

Adds a `-version` or `--version` option to print just the version
of the program for bitcoind, bitcoin-cli and bitcoin-qt.

Also make it that `-help` can be used to display the help (as well as
existing `--help`). Up to now, `-help` was the only option that didn't
work with either one or two dashes.

src/qt/utilitydialog.h

@@ -40,7 +40,7 @@ class HelpMessageDialog : public QDialog
     Q_OBJECT
 
 public:
-    explicit HelpMessageDialog(QWidget *parent);
+    explicit HelpMessageDialog(QWidget *parent, bool versionOnly);
     ~HelpMessageDialog();
 
     void printToConsole();
@@ -48,9 +48,6 @@ public:
 
 private:
     Ui::HelpMessageDialog *ui;
-    QString header;
-    QString coreOptions;
-    QString uiOptions;
 
 private slots:
     void on_okButton_accepted();