Clean up private headers

This commit adds a few missing private headers, and cleans up
some irregularities in the existing ones
overlay: deal gracefully with main widget being absent

It can happen that we get a size request when the main widget
is still NULL. Currently we hit a critical in this case, and
stumble on. We can do better.
Opening a new tab in nautilus is hitting this case.
https://bugzilla.gnome.org/show_bug.cgi?id=727643
Make GtkAccelGroupEntry public again

Even though it was marked as internal, it seems gnome-terminal was
poking into the structure. It's unfortunately part of our ABI.
Set use-fallback property to True for GtkScaleButton icons.

https://bugzilla.gnome.org/show_bug.cgi?id=727662
Bump version to 3.13
Add version macros for 3.14
listbox: Add multi-selection API

This commit adds API for dealing with multi-selection. It is identical
to the flow box API for this purpose. The implementation is still limited
to single-selection, and will be updated in subsequent commits.
list box: Implement multiple selection

This largely copies the flox box implementation, including the
same key bindings.
list box: Update accessible implementation

Now that multi selection is supported, we can provide a more
complete AtkSelection implementation.
Add a list box selection testcase
list box: Cosmetic changes

Shorten variable names from list_box to box, throughout,
and use the BOX_PRIV macro everywhere.
Improve list selection example

Print out signals, so we see how often they are emitted.
list box: Avoid excessive signals

We were emitting the a11y ::selection-changed signal much more
often than ::selected-rows-changed. Thats not necessary.
Add new listbox api to docs
switch: Add a delayed state capability

This commit makes it possible for GtkSwitch to indicate when
the underlying state changes with a delay, causing the switch
to temporarily go 'out of sync' with the underlying change.

https://bugzilla.gnome.org/show_bug.cgi?id=725648
Add a delayed state test case

This adds a switch with delayed state change to testswitch,
demonstrating the functionality that was added in the previous
commit.
Add new switch api to the docs
Deprecate gtk_application_add_accelerator

The new gtk_application_set_accels_for_action api is more flexible
and better.
headerbar: Use popover for the fallback app menu

https://bugzilla.gnome.org/show_bug.cgi?id=723963
doap: update URLs