[py]qtwebengine: meta.platforms; use in qutebrowser, calibre

[ghost]

Description of changes

This four-patch series does the following:

1. qtwebengine: implement meta.platforms correctly, based on the current state as of qtwebengine 5.15.8. Previously, meta.platforms was simply set to platforms.unix, which is certainly not correct. In particular, powerpc64le is not supported right now (but see below)

2. pyqtwebengine: do not claim enablement for a platform which is unsupported by qtwebengine.

3. qutebrowser: do not claim enablement on platforms where the underlying backend (pyqtwebkit or pyqtwebengine) is not enabled. This is straightforward; if the chosen backend does not work then qutebrowser cannot do much.

4. calibre: on platforms where qtwebengine is not enabled, simply omit qtwebengine rather than failing the build. Most of calibre's functionality does not require a web browser, so disabling calibre simply because qtwebengine is unavailable is overaggressive.

As of this moment qtwebengine is not supported on powerpc64le. However that is likely to change soon; the upstream chromium has a full javascript JIT for powerpc64le (which is already used in nodejs on powerpc64le), and gentoo is shipping a usable qtwebengine on ppc64:

https://packages.gentoo.org/packages/dev-qt/qtwebengine

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • powerpc64le-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

[AndersonTorres]

Everything is OK. Now rebase/squash the commits, cleaning them up (no need for those meta-comments).

[ghost]

Squashed back to four commits. Let me know if you'd like it all squashed to a single commit.

[AndersonTorres]

LGTM, LGTBorg.