girparser: grok "nullable"

Understand the new "nullable" attribute in .gir files.

Presently, this is mostly an alias for allow-none='', but it will also
allows a new feature: we can explicitly mark out parameters as having a
nullable type (as a distinct concept from accepting 'NULL' as a
parameter to the C function call in order to ignore the result).

.gir may eventually want to remove allow-none='' some day, so make sure
we also accept nullable='' in all places that we accepted allow-none=''
to mean 'can be NULL'.

https://bugzilla.gnome.org/show_bug.cgi?id=660879
pango: update to latest git
glib-2.0: Allow null hooks for OptionGroup

Allow null pre-parse, post-parse, error and translation hooks.

Fixed bug #729954.
xcb: Set correct C name for Xcb.ScreenIterator.next

https://bugzilla.gnome.org/show_bug.cgi?id=730086
xcb: Fix visuals iterator methods

https://bugzilla.gnome.org/show_bug.cgi?id=730090
xcb: Add binding for xcb_connection_has_error

https://bugzilla.gnome.org/show_bug.cgi?id=730088
xcb: Fix depth iterator methods

https://bugzilla.gnome.org/show_bug.cgi?id=730089
xcb: Add missing fields to Xcb.Setup, Xcb.Screen and Xcb.VisualType

https://bugzilla.gnome.org/show_bug.cgi?id=730091
xcb: Add methods to access pixmap formats

https://bugzilla.gnome.org/show_bug.cgi?id=730092
xcb: Remove unnecessary padding fields

Some structures allow access to the padding fields from the C structures. Since
these don't do anything except make the memory be set out correctly there's no
reason to access them from Vala.

https://bugzilla.gnome.org/show_bug.cgi?id=730094
xcb: Missing EnterNotifyEvent, LeaveNotifyEvent, KeymapNotifyEvent, VisibilityNotifyEvent, ReparentNotifyEvent, GravityNotifyEvent, CirculateNotifyEvent, PropertyNotifyEvent, SelectionNotifyEvent, ColormapNotifyEvent, MappingNotifyEvent classes

https://bugzilla.gnome.org/show_bug.cgi?id=730097
xcb: Add Xcb.Connection.get_atom_name

https://bugzilla.gnome.org/show_bug.cgi?id=730099
Add webkit2gtk-3.0 and webkit2gtk-web-extension-3.0 bindings.

Fixes bug 676249.
xcb: Add Xcb.Connection.create_pixmap

https://bugzilla.gnome.org/show_bug.cgi?id=730103
xcb: Add drawing API

https://bugzilla.gnome.org/show_bug.cgi?id=730148