Introduce a Qt-free JSON implementation

This is essentially QJson with Qt replaced by std:: features.

This is useful to have in circumstances where a Qt dependency
is undesirable, e.g. for the Qt Creator debugger protocol
implementation in our CDB extension.

Change-Id: Iec79c6b23b1e717ce1b6f4d38755287d1f479c13
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>