Deprecate GtkComboBoxEntry in favor of added properties to GtkComboBox

Deprecate GtkComboBoxEntry in favor of added properties to GtkComboBox

GtkComboBox now sports a construct-only "has-entry" property which
decides if it uses a GtkEntry to allow additional user input. Also
it has a new "entry-text-column" to fetch strings for the entry
from the model.

This patch deprecates the GtkComboBoxEntry and updates the rest of GTK+
to use the new semantics on GtkComboBox instead.

GtkComboBoxEntry will be removed altogether before GTK+ 3, in a
later commit.