use either name.size.ext or name-size.ext for all symbol files

[matkoniecz]

extracted from #2042 (comment)

[matthijsmelissen]

I noticed that some other open-source projects have an 'easy' label for tickets that are easy to pick up for new contributors. I think labelling such tickets is a great way to make it easier for people to start contributing to open-source projects, so I'd like to start doing this in our repository as well.

This issue seems suitable, so I added the easy label to it.

[kocio-pl]

If someone is going to pick it up, then go ahead, I would not object, but if not, I'm self assigned to do this (mainly cleaning my mess).

[matthijsmelissen]

Don't worry, it has always been a mess, long before you started with the files :)

[kocio-pl]

One could also play with SVG internals (like layer names and ids) or moving all 16px version to 14px.