Introduce DeviceFileSystemModel

This is going to replace SftpFileSystemModel.

That's needed before planed removal of SshConnection in order
to not to use SftpSession class.

Change-Id: I97f0ffd3418f7e71819ac95048a75e4a17901c71
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/loadcoredialog.cpp

@@ -139,6 +139,7 @@ void SelectRemoteFileDialog::attachToDevice(Kit *k)
     IDevice::ConstPtr device = DeviceKitAspect::device(k);
     QTC_ASSERT(device, return);
     SshConnectionParameters sshParams = device->sshParameters();
+    // TODO: change into setDevice()
     m_fileSystemModel.setSshConnection(sshParams);
 }