feat(example): add RemoteTerm example target and Qt6 fixes

[yangwei-x]

• Add optional RemoteTerm example (BUILD_EXAMPLE_REMOTETERM) with CMake target 'remoteterm'
• Use QStringLiteral for usage message in RemoteTerm main.cpp for Qt6 compatibility.

[tsujan]

@yan12125
Could you review it? It's mainly about README and cmake.

[yan12125]

Some thoughts after a quick glance. Didn't test them yet

[yan12125]

Currently there is only PyQt binding, no official one for PySide yet.

[yan12125]

Ditto for PySide

[yan12125]

Personally I'm not a fan of -j, which often makes the system hang, especially on not-so-powerful machines.

[yan12125]

Minor: PyQt5 is no longer supported. Only PyQt6 is supported, so it is not only recommended, but also needed for the Python example.

[yan12125]

Running remoteterm gives this:

qt.core.qobject.connect: QObject::connect: No such signal QTcpSocket::error(QAbstractSocket::SocketError)

Apparently the signal is renamed. Here is a quick fix:

diff --git a/examples/cpp/RemoteTerm/remoteterm.cpp b/examples/cpp/RemoteTerm/remoteterm.cpp
index 551ac25..b48be65 100644
--- a/examples/cpp/RemoteTerm/remoteterm.cpp
+++ b/examples/cpp/RemoteTerm/remoteterm.cpp
@@ -18,7 +18,9 @@ RemoteTerm::RemoteTerm(const QString &ipaddr, quint16 port, QWidget *parent)
         QByteArray data = socket->readAll();
         write(this->getPtySlaveFd(), data.data(), data.size());
     });
-    connect(socket, SIGNAL(error(QAbstractSocket::SocketError)),this,SLOT(atError()));
+    connect(socket, &QTcpSocket::errorOccurred, [this](QAbstractSocket::SocketError){
+        atError();
+    });
 
     // Here we start an empty pty.
     this->startTerminalTeletype();

After this, I got "Connection refused" for a random IP and port as expected. It would be great to have an example for the server to actually test the functionality of this program.