Props: allow flushing the atomic change listener

Trying to narrow down causes of the ‘unregister listeners crashes on
shutdown’ reports.

simgear/props/AtomicChangeListener.cxx

@@ -59,6 +59,12 @@ void AtomicChangeListener::fireChangeListeners()
     listeners.clear();
 }
 
+void AtomicChangeListener::clearPendingChanges()
+{
+    auto& listeners = ListenerListSingleton::instance()->listeners;
+    listeners.clear();
+}
+
 void AtomicChangeListener::valueChangedImplementation()
 {
     if (!_dirty) {

simgear/props/AtomicChangeListener.hxx

@@ -52,7 +52,17 @@ public:
     bool isDirty() { return _dirty; }
     bool isValid() { return _valid; }
     virtual void unregister_property(SGPropertyNode* node) override;
+
     static void fireChangeListeners();
+
+    /**
+     * @brief Ensure we've deleted any pending changes. 
+     * 
+     * This is important in shutdown and reset, to avoid holding
+     * property listeners around after the property tree is destroyed
+     */
+    static void clearPendingChanges();
+
 private:
     virtual void valueChangedImplementation() override;
     virtual void valuesChanged();

simgear/structure/SGAction.cxx

@@ -0,0 +1,86 @@
+
+
+#if 0
+
+set bindings for action seperate from defintion ?
+    - in XML, especially aircraft XML
+
+define actions from command / Nasal
+add behaviours from Nasal 
+
+define keymapping
+    - manager of keybindings defined against actions?
+    - for a toggle or enum, define behaviour
+        - each key repeat cycles
+        - alternate key to go the other way (G/shift-G)
+
+release bindings for momentary actions:
+    button up / key-up
+
+send activate, release
+
+    send deactivate, release for 'alternate' action
+
+#endif
+
+
+void SGAction::setValueExpression()
+{
+    // watch all the properties
+}
+
+void SGAction::setValueCondition()
+{
+    // 
+}
+
+void SGAction::updateProperties()
+{
+    //
+    _node->setBoolValue("enabled", isEnabled());
+    switch (_type) {
+    case Momentary:
+    case Toggle:
+        _node->setBoolValue("value", getValue());
+        break;
+
+    case Enumerated: 
+        if (!_valueEnumeration.empty()) {
+            // map to the string value
+            _node->setStringValue("value", _valueEnumeration.at(getValue()));
+        } else {
+            // set as an integer
+            _node->setIntValue("value", getValue());
+        }
+    }
+
+    // set description
+}
+
+bool SGAction::isEnabled()
+{
+    if (_enableCondition) {
+
+    } else {
+        return _enabled;
+    }
+
+    updateProperties();
+}
+
+int SGAction::getValue()
+{
+    if (type == Enumerated) {
+        if (_valueExpression) {
+            // invoke it
+        }
+    } else {
+        if (_valueCondition) {
+            return _valueCondition.test();
+        }
+    }
+
+    return _value;
+}
+
+// commands enable-action, disable-action