Handle belowNormalPriority inside process launcher on Windows

Remove setCreateProcessArgumentModifier() from the ProcessInterface
API and replace it with belowNormalPriority flag.

Change-Id: I6bcb92e56c3a68af7fa3e3a1c8b8eb13e3a2e5a7
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/launchersocket.cpp

@@ -409,6 +409,7 @@ void LauncherHandle::doStart()
     p.writeData = m_writeData;
     p.channelMode = m_channelMode;
     p.standardInputFile = m_standardInputFile;
+    p.belowNormalPriority = m_belowNormalPriority;
     sendPacket(p);
 }