Project Management: Switch to Welcome Screen on "Close All Projects"

The natural thing when closing all Projects is to start a new work, so
switching to Welcome Screen seems useful.

Change-Id: I0520848739049f1685075eeed3ccaa17bf143618
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/projectexplorer/projectexplorer.cpp

@@ -1082,6 +1082,8 @@ void ProjectExplorerPlugin::closeAllProjects()
 
     d->m_session->closeAllProjects();
     updateActions();
+
+    Core::ModeManager::activateMode(Core::Id(Core::Constants::MODE_WELCOME));
 }
 
 void ProjectExplorerPlugin::extensionsInitialized()