Expanded PNG export functionality.

Expanded PNG export functionality a bit:

* after pressing the "PNG" button, a "save-as" dialog is shown with a suggested file name (which does not exists yet);
* when the PNG file is created, Java will try to open the folder the image was created in. If this fails, a popup with a message that the image is saved will be shown (as before).

Note that the opening of the folder by the OS' file manager is only tested on Ubuntu 12, where I had to add the line `x-directory/normal=nautilus.desktop` to the file `~/.local/share/applications/defaults.list` to work properly. I believe previous Ubuntu versions will have this by default. Windows and Mac machines should work out of the box though.