Debugger: Fix switching to previous mode on exit

This behavior was broken with refactoring done in commit
3b5ecac238. This has two main components:
1. Perspective::select() needs to call EngineManager::activateDebugMode()
   in order to save the previous mode.
2. The contents of the previous function
   DebuggerPluginPrivate::activatePreviousMode() was placed in
   EngineManager::deactivateDebugMode() and is called in
   doFinishDebugger().

Task-number: QTCREATORBUG-21415
Change-Id: Ibca188ba740027769c497e25ea695af8e218ea4e
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/debuggerengine.cpp

@@ -418,6 +418,8 @@ public:
         m_watchHandler.cleanup();
         m_engine->showMessage(tr("Debugger finished."), StatusBar);
         m_engine->setState(DebuggerFinished); // Also destroys views.
+        if (boolSetting(SwitchModeOnExit))
+            EngineManager::deactivateDebugMode();
     }
 
     void scheduleResetLocation()