Fix string conversion issues with emoji characters (#120)

* Fix load/cast of QString to avoid issues with surrogate pairs (emoji characters, ...).

---------

Co-authored-by: Mikaël Capelle <capelle.mikael@gmail.com>

tests/python/test_qt.cpp

@@ -11,6 +11,14 @@ PYBIND11_MODULE(qt, m)
 {
     // QString
 
+    m.def("create_qstring_with_emoji", []() {
+        return QString::fromUtf16(u"\U0001F600");
+    });
+
+    m.def("consume_qstring_with_emoji", [](QString const& qstring) {
+        return qstring.length();
+    });
+
     m.def("qstring_to_stdstring", [](QString const& qstring) {
         return qstring.toStdString();
     });

tests/python/test_qt.py

@@ -10,10 +10,19 @@ def test_qstring():
 
     assert m.qstring_to_stdstring("éàüö") == "éàüö"
     assert m.stdstring_to_qstring("éàüö") == "éàüö"
+    assert m.qstring_to_stdstring("خالد") == "خالد"
+    assert m.qstring_to_stdstring("🌎") == "🌎"
 
     assert m.qstring_to_int("2") == 2
     assert m.int_to_qstring(2) == "2"
 
+    emoji = m.create_qstring_with_emoji()
+
+    assert emoji.encode("utf-16be", "surrogatepass") == b"\xd8\x3d\xde\x00"
+    assert m.consume_qstring_with_emoji(emoji) == 2
+
+    assert m.consume_qstring_with_emoji("🌎") == 2
+
 
 def test_qstringlist():
     assert m.qstringlist_join([""], "--") == ""