Simplify and improve main application object creation

• useful for re-using the code for tests
• make KexiStartupData singleton acessible, it's the same as KexiStartupHandler::global()
• put QApplication into main(), don't create in KexiMainWindow::create()
• add KexiTestHandler that helps to create Kexi-specific tests
• port GlobalSearchTest to theKexiTestHandler; this makes it a regular QtTest with optional --loop option to stay in even loop