Switch to window takes a handle

[andrewnicols]

https://www.w3.org/TR/webdriver/#switch-to-window

Let handle be the result of getting the property "handle" from the parameters argument.

Apologies but I'm not quite sure how best to unit test this.

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[andrewnicols]

CLA now signed.

[OndraM]

Hi and thanks for the PR! But we need to take care of both protocols and not break things :).

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[andrewnicols]

As I've now come across a second function in the RemoteTargeLocator which needs to maintain b/c, I've passed the isW3cCompliant argument as an optional 3rd argument to the constructor.

This change is on a separate commit shared between this issue and #684

[OndraM]

Nice, thanks! But let me please leave this PR open for just a while - I'd like to add some tests before merging this, because we are obviously missing them :).