When creating a binding between two object properties we might want to
automatically synchronize the two values at the moment of the binding
creation, instead of waiting for the next change.
The G_BINDING_SYNC_CREATE flag does exactly what it says on the tin.
https://bugzilla.gnome.org/show_bug.cgi?id=622281
GBinding is a simple, opaque object that represents a binding between a
property on a GObject instance (source) and property on another GObject
instance (target).
https://bugzilla.gnome.org/show_bug.cgi?id=348080