Replace deprecated functions

Following functions are deprecated in new GTK, replacing them with still
working counterparts:

* gtk_widget_modify_bg
* gtk_widget_modify_fg
* gtk_widget_modify_base
* gtk_widget_modify_text
* gtk_widget_render_icon

Counterparts are:

* gtk_widget_override_background_color
* gtk_widget_override_color
* gtk_widget_render_icon_pixbuf