XBL companion destroy method to be called when switching wizard page

[avernet]

The destroy method should be called because:

• A component with javascript-lifecycle might want to do its own cleanup (e.g. date control unregistering listener).
• For all components, the XForms engine client-side code needs to remove the companion object from the list of active companion objects it keeps track of.
• This will help us reduce memory leaks with embedding (Client-side embedding #1577).

[ebruchez]

Currently, fr|tinymce doesn't support the javascript-lifecycle mode. destroy() is only called when that mode is present. So this explains that.

One way to go would be to move this component (and maybe others) to use javascript-lifecycle.

[ebruchez]

Trying this with the Wizard:

• S1 with a text control
• S2 with an fr:number (control-2) control (which has javascript-lifecycle)
• S1 → S2: xxf:init initializes the control
• S2 → S1: nothing is in the response for control-2
• S1 → S2: xxf:init again