Doing this via the product condition silently switches the product off
if Qt is too old, which will leave users wondering about the reason.
Instead, use the long-supported version check functionality of the
Depends item, which will result in a clear message in case of a failure.
Also, move the check to QtcProduct. The version requirement is for all
of QtCreator, not just the plugins.

Change-Id: Id445823e0243f5ad3b7a0ccb747a90e4fb18889b
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
Reviewed-by: Jake Petroules <jake.petroules@qt.io>