Core: ProcessReaper code cosmetics

- Rename ReaperPrivate to ProcessReapers
- drop the used ProcessReaper::m_futureInterface
- sprinkle some private and final
- remove unnecessary Q_OBJECT
- remove unnecessary #include, fix accidental user

Change-Id: I2787815975dc80cb93db2132bbfe7ce9a74ebd5f
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/coreplugin.h

@@ -87,7 +87,7 @@ private:
     MainWindow *m_mainWindow = nullptr;
     EditMode *m_editMode = nullptr;
     Locator *m_locator = nullptr;
-    ReaperPrivate m_reaper;
+    ProcessReapers m_reaper;
 };
 
 } // namespace Internal