#192 Option to export an image from the Open dialog

[William Kendrick]

Add an option to the "Open" dialog to allow exporting (copying, basically) a single image to an easy-to-access location. (e.g., "~/Pictures" on Linux).

We probably want this to be a configurable location.

See also an idea for exporting a series of images as an animated GIF, from the "Open" dialog's "Slideshow" sub-dialog (https://sourceforge.net/p/tuxpaint/feature-requests/191/).