Fix the minimum size of help text windows to the content size

This could make  help text windows bigger than the  screen, if we want
to  go there  then  using  a QLabel  sub-class  will  need to  change,
probably  by using  a  read-only QTextEdit  instead  as that  provides
scroll bars. Maybe  consider a multi-column table for  the contents as
an alternative to scroll bars.

widgets/HelpTextWindow.cpp

@@ -22,10 +22,11 @@ HelpTextWindow::HelpTextWindow (QString const& title, QString const& file_name,
                                    , tr ("Error: %1").arg (source.errorString ()));
       return;
     }
-  setText (QTextStream {&source}.readAll ());
   setWindowTitle(QApplication::applicationName () + " - " + title);
   setMargin (10);
   setBackgroundRole (QPalette::Base);
   setAutoFillBackground (true);
   setStyleSheet (font_as_stylesheet (font));
+  setText (QTextStream {&source}.readAll ());
+  setMinimumSize (sizeHint ());
 }