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 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 – D14588
• 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).