No horizontal scrollbar on generated images

[mmoser18]

When writing a PUML file the graphic preview (right pane) is often not very useful. Typically only a small part of a graph is shown and the bottom part is often chopped off, even though there would be enough space below the graph - see the attached screenshot . And - while there is occasionally a vertical scroll bar - there NEVER is a horizontal scrollbar, even though the graphs are often 2-3x as wide as the displayed part. So it's impossible to inspect parts of a graph that are outside the visible part.