APT MultiArch support. On multiarch-enabled systems it:

- Prevents foreign-arch packages not explicitly supporting multiarch from being added to
  QApt::Backend's central package list
- Provides a Backend::isMultiArchEnabled() to return whether or not dpkg/apt are set up
  for multiarch
- Provides a Package::architecture() to determine the architecture of a package.
- Provides a Package::isMultiArchEnabled() to return whether or not a package explicitly
  supports multiarch installation.

I am considering this a bugfix, since with APT 0.8.16+ behavior of clients that do not
have a way to distinguish foreign arch packages from native packages to the user are
quite obnoxious, displaying two identical packages with no sane way to tell them apart,
doing this for just about every package available. With this patch, consumers will not
list foreign arch packages that aren't multiarch-enabled, but they will still have to
use the two new QApt::Package functions to disambiguate packages that are multiarch-
enabled.

CCBUG: 276389
FIXED-IN:1.2.1,1.2.65