D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/kde/kf6-kguiaddons/source/examples/python/kdatevalidator.py

@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+
+# SPDX-FileCopyrightText: 2025 Nicolas Fella <nicolas.fella@gmx.de>
+# SPDX-License-Identifier: LGPL-2.1-only OR LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL
+
+import sys
+from PySide6 import QtCore, QtWidgets, QtGui
+from KGuiAddons import KDateValidator
+
+class MyWidget(QtWidgets.QWidget):
+    def __init__(self):
+        super().__init__()
+
+        self.input = QtWidgets.QLineEdit()
+        self.input.editingFinished.connect(self.onEditFinished)
+
+        validator = KDateValidator()
+        self.input.setValidator(validator)
+
+        self.layout = QtWidgets.QVBoxLayout(self)
+        self.layout.addWidget(self.input)
+
+    @QtCore.Slot()
+    def onEditFinished(self):
+        print("Got valid input")
+
+if __name__ == "__main__":
+    app = QtWidgets.QApplication([])
+
+    widget = MyWidget()
+    widget.resize(800, 600)
+    widget.show()
+
+    sys.exit(app.exec())