userMenu: Force reload of background-image on icon changes

When changing the user's avatar image, AccountsService will
overwrite the old image with the new one, so the location
returned by get_icon_file() is always the same.
In order to pick up the change, we need to make sure to clear the
previous image from both StTextureCache and StThemeNode's paint
cache.

https://bugzilla.gnome.org/show_bug.cgi?id=679268