We don't need to use dzl_clear_pointer() anymore now that GLib has access
to g_clear_pointer() that performs similar checks as dzl_clear_pointer().

This ensures that we start hiding functions that do not have our export
macros applied.

Fixes #4

This allows us to animate the bold status of the widget, reducing the
jitter in transitioning to the "sunk" position in the header.

If we had a window that was offset from it's parent, our animation
positioning could be off. This improves that by doing things a bit more
manually.

0 is already the lower bound, with unsigned. Just use MIN().

This uses the mm of the monitor to determine the optimal animation
duration. We have a hatch of 500msec match to avoid getting too
annoying.

We might want to have "classifications" for duration such as
DURATION_FAST, DURATION_NORMAL, DURATION_SLOW, etc

This also switches back to "themes" instead of "css" for the theming
directory. I find that a bit easier to remember.

This moves a bunch of the theming stuff into a data directory which will
hopefully be easy to get contributors to add new theming support. One of
the main reasons Builder looks so poor on non-Adwaita is that we need to
encourage themes to help us out here, given all the custom widgets.

Although, maybe with dazzle broken out of Builder, it will be easier for
them to just support it from their theming files.

This assumes image|label in a box, but it's better than nothing for now.

This widget provides what we are calling a "Stack List". It
allows for diving down into a hierarchical data set similar to
a treeview, but without some of the difficulties of a treeview.

StackList does not indent as you dive down. You can only look
at a single parent in the hierarchy at a time. Generally, this
is what we want when we mistakenly use a treeview in a sidebar.

To be extra dazzling, we animate the row transition as we dive
to the next layer of the hierarchy.

This widget uses GListModel for content. Having a GListModel
of items which are also a GListModel is an ideal way to use
this widget. However, you probably want lazy initialization with
GAsyncIntiable for the descendants.