Can't set path to a platform plugin in a portable way

Currently in Qt5 there is no way how to set a path to a platform plugin in a
portable way for a deployment environment - due to restriction that platform plugin needs to be found already during creation of QApplication object.

Proposed solution is:

static void init()
{
QString plugindir;
#ifdef Q_OS_MAC
plugindir=QCoreApplication::applicationDirPath()+"/../../Plug-Ins"; // MyApp.app/Contents/MacOS/../../Plug-Ins
#elif defined Q_OS_WIN
plugindir=QCoreApplication::applicationDirPath()+"\\plugins"; // MyApp\bin\plugins
#elif defined Q_OS_LINUX
plugindir=QCoreApplicaton::applicationDirPath()+"/../plugins"; // MyApp/plugins
#endif
putenv( "QT_PLUGIN_PATH", plugindir );
}

int main(int argc, char *argv[] )
{
init();
QApplication a(argc, argv);
….
}

If someone moves the whole application structure (Mac OS bundle for example), then plugin path still would be resolved correctly.

This would require QCoreApplication::applicationDirPath() to be truly static. Currently it depends on QCoreApplication instance. Its easy to remove that requirement in applicationDirPath() and applicationFilePath() [which is called by applicationDirPath() ].

The issue is that we can't remove dependency to "self" in QCoreApplication::arguments(), which is called from applicationFilePath() (for blackberry and Q_OS_UNIX). It would be possible to make it truly static
for Mac, Win, Linux.