GlanceMirror class does not expose progress update callbacks

The GlanceMirror class has no callback to provide progress updates on
image downloads. OS images are often large enough (and you may be
syncing several images) that this presents a serious UX problem during
which a user (or client program) of simplestreams will not be able to
tell that anything is happening.

The attached diff exposes progress update callbacks with a minimum of
changes to existing program logic. The GlanceMirror class gets a basic
callback, and calling code is expected to handle figuring out what
images will be downloaded and their size before calling
GlanceMirror.sync(). In order to help with this, a separate class
"ItemInfoDryRunMirror" is added, to make use of the same filter logic
that GlanceMirror will use but instead just collect a list of images
to download and their size.

Since this is provided as optional keyword arguments, existing users
will not be affected.

[Test Case]

From the source tree top directory,

PYTHONPATH=. tools/sstream-mirror-glance --max=1 -v -v -v --output-dir \
images/ --cloud-name test_cloud --content-id test_content \
--keyring /usr/share/keyrings/ubuntu-cloudimage-keyring.gpg \
http://cloud-images.ubuntu.com/releases/ streams/v1/index.json

And verify that it prints status updates to stdout. The
sstream-mirror-glance tool has been updated to use the new api in
GlanceMirror.

[Regression Potential]

This has limited regression potential, because the new functionality
is provided as optional arguments. If anything is running the
sstream-mirror-glance script and parsing its output, that may break,
but that seems very unlikely because that script does not currently
produce anything useful to parse.