Add option to reuse remapped images

[Joachim Schneider]

When using enblend/enfuse it can happen that there is already a set of remapped images, e.g. when enblend has aborted the remapped images are not deleted. In the next run you get asked if the existing images should be overwritten. You should get the alternative to skip remapping and use them for enblend/enfuse directly. A checkbox for deleting them afterwards would also be helpful.

[Bruno Postle (brunopostle)]

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If you stitch on the command-line using `make -f project.pto.mk` then existing intermediate files will be reused.

The upcoming batch processor Summer of Code project should support this sort of thing in the GUI, untested.