Merge "Set Property" undo commands if they affect the same property

Closes #3103

src/tiled/changeproperties.cpp

@@ -104,6 +104,27 @@ void SetProperty::redo()
         mDocument->setProperty(obj, mName, mValue);
 }
 
+bool SetProperty::mergeWith(const QUndoCommand *other)
+{
+    // If the same property is changed of the same layer, the commands can
+    // be trivially merged. The value is already changed on the layer, and
+    // the old value already remembered on this undo command.
+    auto o = static_cast<const SetProperty*>(other);
+    if (mDocument == o->mDocument && mName == o->mName && mObjects == o->mObjects) {
+        mValue = o->mValue;
+
+#if QT_VERSION >= QT_VERSION_CHECK(5, 9, 0)
+        setObsolete(std::all_of(mProperties.cbegin(), mProperties.cend(),
+                                [this] (const ObjectProperty &p) {
+            return p.existed && p.previousValue == mValue;
+        }));
+#endif
+
+        return true;
+    }
+    return false;
+}
+
 
 RemoveProperty::RemoveProperty(Document *document,
                                const QList<Object*> &objects,

src/tiled/changeproperties.h

@@ -21,6 +21,7 @@
 #pragma once
 
 #include "object.h"
+#include "undocommands.h"
 
 #include <QString>
 #include <QUndoCommand>
@@ -79,6 +80,10 @@ class SetProperty : public QUndoCommand
     void undo() override;
     void redo() override;
 
+    int id() const override { return Cmd_SetProperty; }
+
+    bool mergeWith(const QUndoCommand *other) final;
+
 private:
     struct ObjectProperty {
         QVariant previousValue;

src/tiled/undocommands.h

@@ -48,6 +48,7 @@ enum UndoCommands {
     Cmd_ChangeWangSetName,
     Cmd_EraseTiles,
     Cmd_PaintTileLayer,
+    Cmd_SetProperty,
 };
 
 /**