Move SessionManager to Core plugin

Sessions are independent from projects, and this allows for removal of
e.g. the dependency from Bookmarks to ProjectExplorer.

Includes moving the command line arguments (-lastsession, <session>) to
Core plugin.

Change-Id: I6c578fd15c4990902e7196501de20f39376e90e8
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/coreplugin/coreplugin.h

@@ -9,6 +9,8 @@
 #include <extensionsystem/iplugin.h>
 #include <utils/environment.h>
 
+#include <memory>
+
 QT_BEGIN_NAMESPACE
 class QMenu;
 QT_END_NAMESPACE
@@ -20,6 +22,7 @@ class PathChooser;
 namespace Core {
 
 class FolderNavigationWidgetFactory;
+class SessionManager;
 
 namespace Internal {
 
@@ -74,6 +77,7 @@ private:
     MainWindow *m_mainWindow = nullptr;
     EditMode *m_editMode = nullptr;
     Locator *m_locator = nullptr;
+    std::unique_ptr<SessionManager> m_sessionManager;
     FolderNavigationWidgetFactory *m_folderNavigationWidgetFactory = nullptr;
     Utils::Environment m_startupSystemEnvironment;
     Utils::EnvironmentItems m_environmentChanges;