Prevent ClickableComponents re-adding event listeners each time enabled() is called.
* Keeps track of enabled state (this.enabled_)
* enable() doesn't do anything if the component is enabled, so the event handlers are not re-added
Fixes#4312
enable() and disable() on clickable components is only cosmetic. "Disabled" implies the control should not be functional.
* Remove event listeners on disable() and add back on enable().
* Move adding listeners from constructor to enable
* Remove tabindex from disabled components and add disabled attribute to disabled buttons to prevent keyboard access.
