[warningectomy] const correctness for diagram_add_update_with_border

also for ddisplay_add_update_with_border()
[gtk-x11] fallback font context for running console version

build --without-freetype can not run unit tests because of missing
font context. But to have rotatable text it's currently advised to
not use freetype directly.
[warningectomy] one more cast to xmlChar*

render_svg.c:614:62: warning: passing 'char [6]' to parameter of type 'const xmlChar *' (aka 'const unsigned char *') converts between pointers to integer types with different sign [-Wpointer-sign]
    g_return_if_fail (node != NULL && xmlStrcmp (node->name, "image") == 0);
                                                             ^~~~~~~
[warningectomy] remove unused function _toggle_data_func()

prop_sdarray_widget.c:95:1: warning: unused function '_toggle_data_func' [-Wunused-function]
_toggle_data_func (GtkTreeViewColumn *tree_column,
^
[warningectomy] prototype for function 'diagram_tree_show'

diagram_tree_view.c:556:1: warning: no previous prototype for function 'diagram_tree_show' [-Wmissing-prototypes]
diagram_tree_show (void)
^