Allow mass-saving and editing opened images

[kamilburda]

Exporting images opened in GIMP was originally slated to be a part of batch file conversion. I have since decided to separate exporting the opened images into a standalone procedure with its own dialog for easier discoverability and slightly less complexity in achieving this task by users.

Additionally, we should allow toggling between exporting (not overriding the existing opened images), saving the opened images as XCF files (saving any unsaved changes in the process) and editing the opened images. Alternatively, saving and editing should be separate from exporting, just like saving and exporting a single image is separated in GIMP.

[kamilburda]

Exporting images has been implemented earlier than expected and will be included in 1.0. This was made possible easily as I already had the codebase prepared for handling opened images.