Doc: document QML emulation layer
Doc: document QML emulation layer

And the new option to have Qt Creator build the emulation
layer using the kit selected for the project.

Change-Id: I16b3d9e87f21caf7a4ecd1a527df51035494fb34
Reviewed-by: Thomas Hartmann <588ee739c05aab7547907becfd1420d2b7316069@digia.com>

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