[balsa/gtk4: 114/289] Remove LibBalsaCellRendererButton

[Peter Bloomfield <peterb src gnome org>]

commit 399d212d3b5973e2a60a68752ff9a35f3d3b471b
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Wed Sep 23 16:52:36 2020 -0400

    Remove LibBalsaCellRendererButton
    
    We only needed it to capture clicks, which we can do with GtkTreeView's
    "row-activated" signal.
    
    * libbalsa/cell-renderer-button.c: removed;
    * libbalsa/cell-renderer-button.h: removed;
    * libbalsa/address-view.c: use "row-activated" instead.