Rewrite .contentDestroyed() to be more paranoid.
Rewrite .contentDestroyed() to be more paranoid.

This version is more aggressive and explicit about removing stale menu command objects, which hold references to content windows, and thus can leak memory if never cleared.

Refs #1578

the8472/greasemonkey,Ventero/greasemonkey,Ventero/greasemonkey,HasClass0/greasemonkey,Martii/greasemonkey,HasClass0/greasemonkey,Martii/greasemonkey,the8472/greasemonkey