Port the Open code away from NetAccess.

Summary:
NetAccess::download used a temp file, to then load that with
QImageReader.

I'm using an in-memory buffer instead, mostly because it's
less code. It should also be faster, at the expense of more
memory usage of course (the PNG data should be less than the
image data anyway).

Test Plan: Opening files still works

Reviewers: broulik, andreagenor, tcanabrava, vkrause

Reviewed By: vkrause

Maniphest Tasks: T11576

Differential Revision: https://phabricator.kde.org/D25477