QbsProjectManager: Switch to an out-of-process approach

That is, do not link to the qbscore library anymore. Instead, use the
JSON-based API.
Advantages:
    - We can build Qt Creator with qbs support without qbs being present
      on the build machine.
    - Smaller memory footprint for Qt Creator, as the qbs build graphs
      are now being managed by a separate process.
    - Potential crashes in qbs will not kill the Qt Creator process.

Fixes: QTCREATORBUG-20622
Task-number: QTCREATORBUG-22904
Change-Id: If7d344b0ac65a99ff0a3a3db215d61b8d903e47e
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/plugins/qbsprojectmanager/qbsnodetreebuilder.h

@@ -25,21 +25,18 @@
 
 #pragma once
 
-#include "qbsnodes.h"
-
-#include <qbs.h>
+#include <memory>
 
 namespace QbsProjectManager {
 namespace Internal {
 
-// ----------------------------------------------------------------------
-// QbsNodeTreeBuilder:
-// ----------------------------------------------------------------------
+class QbsBuildSystem;
+class QbsProjectNode;
 
 class QbsNodeTreeBuilder
 {
 public:
-    static std::unique_ptr<QbsRootProjectNode> buildTree(QbsBuildSystem *project);
+    static std::unique_ptr<QbsProjectNode> buildTree(const QbsBuildSystem *buildSystem);
 };
 
 } // namespace Internal