Currently Spectacle defaults to `Screenshot_%Y%M%D_%H%m%S.png` for the filename template, e.g. `Screenshot_20180111_102030.png`, which is not even translated.
Let's make the first part of the string translatable, and use something more readable for the second part. While [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) as suggested by @acrouthamel in D10709#211149 defines a universal format which also sorts well in a directory, it's still a bit hard to read and includes colons, which are unsupported when saving to FAT32.
Gwenview uses a slightly adapted version of the standard, doing the same for Spectacle would yield `Screenshot_2018-01-11_10-20-30.png`, which seems just fine.
Steps:
[ ] Achieve consensus on the future format
[ ] Deduplicate default filename template code
[ ] Make first part of the filename translatable
[ ] Implement new template
@ngraham Please let me know if you are okay with this, I'll then send Diffs (or commit directly, if you prefer that).