ProjectExplorer: Add BuildSystem class

Add a class to abstract out the BuildSystem from the Project.

Thie idea is to make a Project have-a BuildSystem, so that
it can stop being one.

The Projects in the different ProjectManagers will get much
simpler that way (and many will only consist of a constructor
with some setter calls) and handles all the interactions
between the rest of Qt Creator and the project.

The complex code to interact with the build systems is then
separate from that.

Change-Id: I3bbb000bb1aed91f0b547ac493f743ede8afb8cd
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cmakeprojectmanager/cmakeproject.h

@@ -32,10 +32,8 @@
 #include "cmakebuildtarget.h"
 #include "cmakeprojectimporter.h"
 
-#include <projectexplorer/extracompiler.h>
+#include <projectexplorer/buildsystem.h>
 #include <projectexplorer/project.h>
-#include <projectexplorer/projectmacro.h>
-#include <projectexplorer/treescanner.h>
 
 #include <utils/fileutils.h>
 
@@ -50,8 +48,6 @@ namespace ProjectExplorer { class FileNode; }
 
 namespace CMakeProjectManager {
 
-class BuildSystem;
-
 namespace Internal {
 class CMakeBuildConfiguration;
 class CMakeBuildSettingsWidget;
@@ -66,8 +62,6 @@ public:
     explicit CMakeProject(const Utils::FilePath &filename);
     ~CMakeProject() final;
 
-    BuildSystem *buildSystem() const { return m_buildsystem.get(); }
-
     ProjectExplorer::Tasks projectIssues(const ProjectExplorer::Kit *k) const final;
 
     void runCMake();
@@ -94,11 +88,6 @@ private:
 
     mutable std::unique_ptr<Internal::CMakeProjectImporter> m_projectImporter;
 
-    std::unique_ptr<CMakeBuildSystem> m_buildsystem;
-
-    // friend class Internal::CMakeBuildConfiguration;
-    // friend class Internal::CMakeBuildSettingsWidget;
-
     friend class CMakeBuildSystem;
 };