Exporting tree view image improvements

The following improvements should be done:

1. Scroll bars should be removed from an image exported (see, e.g. the image in the attachment - it contains the scroll bars!). Should be verified for ALL FORMATS!!

2. The default name of the exported image should not contain 2 extensions (like "mega_test2.nwk.jpg"): ".nwk" and e.g. ".jpg" - the ".nwk" extension should be truncated.

3. The "Export Tree Image" item should be added to the context menu.