https://bugzilla.gnome.org/show_bug.cgi?id=710169

Not sure what's happening here, but crashing isn't the thing to do.

Instead of a throbbing button, we now use a separate label
with a blue background for the counter.

https://bugzilla.gnome.org/show_bug.cgi?id=710169

We ship custom HighContrast style, so we should keep it up-to-date.

Until gnome-shell gets switched to use a setting like
we do here, the folders functionality is not really useful,
so don't expose it in the UI unless the TEST_FOLDERS
environment variable is set.

The selected rows may get a new folder when the app folder
dialog is done, so refresh them.

The GsFolders approach requires us to refresh app widgets
'from the outside', so make gs_app_widget_refresh() public.

Otherwise, we end up with the header bar reset, but still
checkboxes in the list.

When the installed list is in selection mode, show a bottom bar
that allows to bring up the app folder dialog.

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.