Add another dialog to test

This one is taken from polari, and currently doesn't work
as expected.
GtkDialog: rejigger headerbar handling

We can't avoid calling set_titlebar in buildable_add_child, since
GtkBuilder assumes that it can e.g. call grab_default at the end,
which only works if the widget is inside a toplevel at that point.

Instead, drop the titlebar and clean up priv->headerbar in constructed
if we find that we don't want a headerbar.
GtkDialog: Be more careful in handling the headerbar

Just because an action widget has a parent thats a headerbar,
it is not guaranteed that it is 'our' headerbar. Add some
assertions to enforce this.
icon-browser: Improve the look a tiny bit

Add some padding and a border around the scrolled window.
gtkwindow: Remove a bogus comment

gtk_window_configure_event certainly doesn't fill the new allocation
in. We're filling it in right here!
gtkwindow: Clean up gtk_window_move_resize a bit more

Don't shadow existing variables. Instead of sharing the allocation and
then overwriting the width/height when convenient, declare it in the
block we use it in, as, really, the three different paths are all
extremely different, and there's no sense in sharing the variable.
gdkwindow-wayland: Take out the margins for now

xdg-shell has moved on and replaced set_margin with set_window_geometry.
To properly support set_window_geometry requires a full rewrite of how
we've been dealing with toplevel windows for now, so just don't set any
margin until we can have a proper toplevel window abstraction in GTK+.
wayland: Update xdg-shell
Trivial cleanup
GtkProgressBar: implement clipping

This allows progress bars to have shadows.

https://bugzilla.gnome.org/show_bug.cgi?id=733361
GtkSwitch: implement clipping

So switches can have shadows, too.

https://bugzilla.gnome.org/show_bug.cgi?id=733360
Adwaita: widget edge for switch and progressbars
GtkLevelBar: implement clipping

So level bars can have shadows, too.
Adwaita: better colorswatches for the dark theme

...which reveals a "nice" border/fill border radius issue.
GtkBox: Set orientable style classes initially

This was an omission, horizontal/vertical would only be set
when the orientation is changed.
GtkFlowBox: Set orientable style classes initially

This was an omission, horizontal/vertical would only be set
when the orientation is changed.
GtkGrid: Set orientable style classes initially

This was an omission, horizontal/vertical would only be set
when the orientation is changed.
GtkPaned: Set orientable style classes initially

This was an omission, horizontal/vertical would only be set
when the orientation is changed.
GtkProgressBar: Set orientable style classes initially

This was an omission, horizontal/vertical would only be set
when the orientation is changed.
GtkRange: Set orientable style classes initially

This was an omission, horizontal/vertical would only be set
when the orientation is changed.