egg-list-box: translate motion-notify events for child windows

When a motion event is received relative to a child window, translate
the y coordinate of the event relative to our own window, since that's what
we need to fetch the correct child.

https://bugzilla.gnome.org/show_bug.cgi?id=691181
build: Fix libtool related warning

warning: 'libegglistbox.la': linking libtool libraries using a non-POSIX
archiver requires 'AM_PROG_AR' in 'configure.ac'

So add it before LT_INIT so it can be used by libtool
egg-list-box: Fix indentation

https://bugzilla.gnome.org/show_bug.cgi?id=691979
egg-list-box: Move refiltered behavior to be a signal

If we want to show a widget when there are no children left after
a refilter, we need a hook to be able to calculate that. Adding
a signal allows us to run handlers before and after the refiltering
is done.

https://bugzilla.gnome.org/show_bug.cgi?id=691979
egg-list-box: Remove redundant check

A ChildInfo's widget should never be NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=691979
Improve keynav

Using gtk_widget_keynav_failed() instead of hardcoding
gtk_widget_error_bell() gives applications a chance to override
keynav handling. We will make use of this in the power panel
to 'link' arrow keynav across multiple list boxes.

https://bugzilla.gnome.org/show_bug.cgi?id=692275
Improve keynav some more

The previous commit made it so that arrow up/down can leave the
list box when the last child is reached. But it did not work for
Page up/down, due to the way the code was written. With this
commit, all up/down key bindings can leave the widget.
A minimal accessible implementation

To inform ATs about what is going on, implement AtkSelection,
and also emit ::active-descendant-changed. This seems enough
to make orca read 'filler' for each row (unless the row happens
to be just a label, in which case, it reads the label).
https://bugzilla.gnome.org/show_bug.cgi?id=692258
Improve a11y implementation

Set the role to ATK_ROLE_LIST_BOX, and add the state
ATK_STATE_MANAGES_DESCENDANTS.
Avoid double emission of active-descentant-changed

egg_list_box_update_selected already calls egg_list_box_update_cursor,
so no need to call the two in a row. This was harmless, but it now
causes extra emissions of a11y signals.
https://bugzilla.gnome.org/show_bug.cgi?id=692258
Add EggFlowBox

Derived from EggWrapBox originally written by
Tristan Van Berkom <tristanvb@openismus.com>
flow-box: add basic selection support
flow-box: use wink instead of sad
flow-box: add pc file
flow-box: add more error checking
flow-box: properly initialize max children per line
flow-box: select child on button release
flow-box: add prelight
flow-box: add basic keynav
Add properties to EggListBox

This makes the widget more useful in glade.
https://bugzilla.gnome.org/show_bug.cgi?id=689711