Protect with mutexes all the struct members that are accessed from
multiple threads, and make sure that signals handled by the view
are emitted in the main thread.

https://bugzilla.gnome.org/show_bug.cgi?id=707185

The default main context can be iterated only from the default thread
(because it includes Gdk/Gtk sources), so it cannot be used from
threadsafe methods.

https://bugzilla.gnome.org/show_bug.cgi?id=707185

Plugins seem to return a bunch of utter nonsense, currently.
Add some assertions to teach them. This is clearly not something
we can ship, but it at least shows whats wrong. We should fix
the plugins to pass these assertions, and then turn them into
a warning, plus filtering out bad apples.

This allows offline applications to be shown in the software center.

We want plugins to be able to set the ID from other metadata items if required.

No need to construct these widgets manually.

Use a list box instead of a tree view, add separators,
disable selection.

Manage spinner visibility using a stack instead of an overlay.
At the same time, add a 'No results found' text to the search
page.

When going back to the OS updates list, the title needs
to be set back to "OS updates".

Add the shadow back, but only if the text is scrolling.

Arrange things so that we scroll to the top only when switching
pages using the main buttons, not when going back from a category
or app page.

Keep track of the current subcategory on the category page,
and go back to that subcategory when coming back from an app
page.

Future-proof this a bit, by also supporting going back
to an app page - this may be used in the future when we
get 'related' links on the app details page.

Replace the popup dialog we had here with going to the
details page. The improved back button handling lets us
go back to the installed page from there.

Add a stack, so we can go back from an application to the category
to the overview.

With the search entry now being only in the overview, we are
always going back to the overview when exiting search, so
no need to keep a search_back_id.

This makes page up/down work when you get to the page.
Also, scroll the overview back to the top every time.

Grabbing focus to a widget is a little tricky, since it
must be mapped for that to work. Add a utility function
that waits for that to happen.

This doesn't make a big difference, but it is the proper
way to show a window.

This improves keynav, and doesn't hurt.

For 3.10, we're just going to hardcode a list of eighteen
popular applications. These were chosen by Allan Day and
Ryan Lerch. We put them on a 3 day rotation, and scramble
them a  bit to make things more interesting. After 3.10,
we will look at getting this data online.

The three apps were chosen by Allan Day and Ryan Lerch. For
3.10, we're just going to hardcode gimp, gnome-weather and
gnome-sudoku as featured apps, and put them on a 3 day
rotation. After 3.10, we will look at getting this data
online.

These will be replaced by proper artwork soon.

This is important for accessibility, and there is really no reason
to treat this button different from all the others.

It is more confusing than helpful, because it might also
filter the installed list.

Too much red hurts the eyes.

Don't make install buttons blue, the mockups don't have that.
And add a way to remove the redness from the remove buttons,
since a wall of red is jarring.