Bug 1020609 - Make configurability check in Xray defineProperty match the spec. r=bz

This code is basically emulating the ES semantics with respect to non-configurable
properties. Non-configurable value properties can still be writable, in which case
their value and writability may be updated.
Bug 1020609 - Inherit property descriptor attributes for pre-existing own value props in Proxy::set. r=efaust
Bug 1020609 - Fix up SpecialPowers for Array Xrays. r=bz

This just makes the special Object behavior apply to Array as well.
Bug 1020609 - Fix up webconsole. r=past

The basic setup is that Object (and now Array) Xrays apply various heuristics
to try to filter out dangerous properties from the underlying object. This
includes accessor properties, shadowing properties, callables, and non-Xrayable
objects. We need to make various fixes on the devtools side to keep this stuff
working.

Also, it would be nice if the tests were designed in such a way that a failed
string match didn't cause the whole test to hang without a useful error message.
Bug 1020609 - Fix up mozApps for Array Xrays. r=mccr8
Bug 1020609 - Implement Xrays to Arrays. r=bz