Different image sorting than by name

[karlch]

Currently images are always sorted by name / path. There could be different sorting, e.g. case-sensitive vs. case-insensitive, by time (exif, access, ...) and so forth.

Implementation could be a new setting image.order or similar, which has different options. The options would be converted to a function that takes the image path an returns the sort key so it can be passed to sorted.

Therefore there would be two main steps:

• Write a new ImageOrder setting
• Use the setting value in the calls to sorted, one in utils.files.listdir for library, opening in the beginning, one in api.commands._CommandArguments.parse_args for path-like commands

Thanks for the initial idea in karlch/vimiv#79