Spotted by clang.

We can use the existing property-changed mechanisms in a smarter way.

This is probably another problem in the stack, but it's helpful to show
something on the console.

Using gnome-software 3.10.x plugins in a 3.11.x binary (and vice-versa) causes
all kinds of mischief.

Additionally, use the full ID to avoid appending .desktop to the name.

This is just cosmetic, the consumers of this just don't care.

We use UNKNOWN to force the state machine into a known state.

g_network_monitor_get_default is transfer-none, so we need
to take a reference if we want to drop it later on.

This moves more code from the gsd updates plugin here: We now
also do the notification at login after an offline update.

At the same time, move the code that deals very specifically
with the internals of offline updates to its own file.

We need to refresh the headers after the list is sorted.

This allows us to easily profile the category searching.

I run xmllint --format to clean things up; you should too.

This is the desired appearance. We use the same style class as
GtkHeaderBar, to remain theme-friendly.

https://bugzilla.gnome.org/show_bug.cgi?id=710629

You need new AppStream metadata for this to work. We'll do something closer to
the mockups soon.

These are not yet used in the Fedora or SUSE metadata files.

This worked before if the unknown tag was in the root <application> but fails
if it appears anywhere like <screenshots> or <keywords>

This allows us to replicate what the UI is doing, for instance:

./gnome-software-cmd search 0ad --show-results --refine-flags=size

For PackageKit this is a list of the PackageIds, but for other plugins it can
just be the desktop filename for example.

This now allows PackageKit to install and remove applications that have multiple
sources.

This allows us to have an AppStream application that has more than one
<pkgname/> tags. This allows us to define meta-applications that consist of more
than one package, for instance a 'font' where each face is in a different package.

This needs a bit more work before things like install and remove work for
applications with multiple sources, but is certainly a small step in the right
direction.