Replace quickfix GIFs by PNGs

[Bananeweizen]

Also add a more specific one for the NeedBraces quickfix.

There is no functional change, just better support for HiDPI displays.

[rnveach]

@Bananeweizen Isn't quickfixes still broken at #315 .

If so, then I would question how you can be sure this doesn't break anything for something that is already broken.

[Calixte]

@Bananeweizen Isn't quickfixes still broken at #315 .

If so, then I would question how you can be sure this doesn't break anything for something that is already broken.

#315 is fixed by #566 so I was able to verify this works.

[Bananeweizen]

@rnveach I often work on branches which contain multiple of the non-merged PRs in between, because often additional features require the non-merged stuff as prerequisite. I remember being 15 (!) commits ahead of master at some point during a Christmas hacking session. And for this change it really was just replacement of file names by other file names, so that was relatively safe. :)

[rnveach]

I forgot that there was a PR sitting for it to be merged.