This will be used in subsequent commits.

Make it possible to show a checkbox at the very left of the
app widget. This will be used to implement selection mode for
the list of installed apps.

We show the folder name on a gray background.

Unlike categories, we use a single, singleton object here that
wraps the app-folders setting and offers an api to make changes
to this setting.

This will eventually be defined in a gnome-shell
schema. For now, we just use a key in our own schema.

The type is a dictionary that maps folder ids to lists
of app ids.

This prevents applications without long descriptions or screenshots from being
crunched up in the middle.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=710632

We can't support changing this, and it's always required only once.

Resovles: https://bugzilla.gnome.org/show_bug.cgi?id=711846

Also add quite a few self tests.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=712256

It's what gnome-packagekit and apper have done for a few releases now.

- the backdrop helps to outline the base of the
  screenshots and thumbnails while they are loading.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=708911

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=710880

If a distro does not provide all the sizes of screenshot required, choose the
image with the size most similar, not just the first one.

This fixes the 'huuuuuge eclipse screenshot' issue.

I've done this for three reasons:

* We already documented how to force notifications for Fedora QA
* Sometimes g_ascii_strtoll() is failing for valid input
* It's not nice to write random files without some kind of schema

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=711503

Side note: an OSTree-like rigorous coding style where ret = FALSE
always at the start of a function would make this error setting errors
not happen.

https://bugzilla.gnome.org/show_bug.cgi?id=711368

This is quite important now gnome-software is running as a service and makes
it much easier to add or modify appstream XML files on a running system.