Show image preview of selected layer

[khalim19]

Quite a few settings now affect how the layer is exported (inserting with back/foregrounds, cropping) and at least one more is on its way. It's easy to get lost how each setting affects the look of the layer once it gets exported.

We need to provide an image preview of the selected layer. The layer will be selected in another preview widget discussed in issue #35.

The implementation may turn out to be difficult depending on whether the preview widgets in PyGIMP's gimpui module are functional and whether pixel operations can be performed fast enough even on Python.

[khalim19]

Done.