False file extention formats for FileChooser in ChartViewer.class

[amassalimov]

When you save an image you need to type file.png in as a file name (not just file)
I guess it should be not "png" but "*.png" etc.

wrong: new FileChooser.ExtensionFilter(
"Portable Document Format (PDF)", "pdf");

right?: new FileChooser.ExtensionFilter(
"Portable Document Format (PDF)", "*.pdf");

[David Gilbert]

Thanks for the report, I committed a fix for this for the next release.