st-theme-context: Punt icon theme changes to an idle

Icon theme change signals aren't noticed immediately, they're usually
noticed when trying to load an icon. Since icon theme changes cause a
style change, and most icon widgets try to re-load their texture during
a style change, this means that we get a stack like this:

  st_texture_cache_load_icon
  gtk_icon_theme_lookup_icon
  gtk_icon_theme_changed
  st_widget_style_changed
  st_texture_cache_load_icon

Rather than making every place that uses StTextureCache re-entrant,
punt the notifying of icon theme changes to an idle handler instead.

https://bugzilla.gnome.org/show_bug.cgi?id=673512