+/** @brief Update the state of one of the control icons
+ * @param button Widget for button
+ * @param visible True if this version of the button should be visible
+ * @param usable True if the button is currently usable
+ *
+ * Several of the icons, rather bizarrely, come in pairs: for instance exactly
+ * one of the play and pause buttons is supposed to be visible at any given
+ * moment.
+ *
+ * @p usable need not take into account server availability, that is done
+ * automatically.
+ */