Implement scenario player

A scenario player may be used for testing Creator crashes which
can't be easily tested with "-test <plugin>" option. Some crashes
are triggered when Creator unloaded plugins and left the
main function. This may happen due to some other threads may
still be running. This scenario can't be tested using plugin tests,
since when the test finishes, Creator still has its plugins loaded.
Also it's not possible to quit Creator from inside the plugin
test, as if we do it, we couldn't report the test result.

The follow up patches will introduce the first test scenario
and provide automatic test for testing against regression
in StringTable.

The scenario player may be potentially used for other purposes,
including automatic presentation of features (yeah!). However,
most probably the API should be further developed for other purposed.
This is just a starting idea.

Change-Id: I0f5c3c028f35a5cdf9130c2cf315dd4b68e81126
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/coreplugin/icore.cpp

@@ -210,6 +210,10 @@ ICore::ICore(MainWindow *mainwindow)
             qWarning("Test run was not successful: %d test(s) failed.", failedTests);
         QCoreApplication::exit(failedTests);
     });
+    connect(PluginManager::instance(), &PluginManager::scenarioFinished, [this] (int exitCode) {
+        emit coreAboutToClose();
+        QCoreApplication::exit(exitCode);
+    });
 }
 
 /*!