Only guard DND cancel events with the version check

In a recent refactor a guard seemingly in the wrong place on
SeatInterface was moved into AbstractDataSource, as typically that's
where we guard.

However it turns out the original code was correct, we want to send
cancel for all clipboard cancels, but for D&D it's version dependent.

This class introduces a new method to astract that from seat.

src/wayland/abstract_data_source.h

@@ -67,6 +67,13 @@ public:
         Q_UNUSED(action);
     };
 
+    /**
+     *  Called when a user stops clicking but it is not accepted by a client.
+     */
+    virtual void dndCancelled()
+    {
+    }
+
     virtual wl_client *client() const
     {
         return nullptr;

src/wayland/datasource_interface.cpp

@@ -118,9 +118,7 @@ void DataSourceInterface::requestData(const QString &mimeType, qint32 fd)
 
 void DataSourceInterface::cancel()
 {
-    if (wl_resource_get_version(resource()) >= WL_DATA_SOURCE_DND_FINISHED_SINCE_VERSION) {
-        d->send_cancelled();
-    }
+    d->send_cancelled();
 }
 
 QStringList DataSourceInterface::mimeTypes() const
@@ -173,6 +171,15 @@ void DataSourceInterface::dndAction(DataDeviceManagerInterface::DnDAction action
     d->send_action(wlAction);
 }
 
+void DataSourceInterface::dndCancelled()
+{
+    // for v3 or less, cancel should not be called after a failed drag operation
+    if (wl_resource_get_version(resource()) < 3) {
+        return;
+    }
+    d->send_cancelled();
+}
+
 wl_resource *DataSourceInterface::resource() const
 {
     return d->resource()->handle;

src/wayland/datasource_interface.h

@@ -41,6 +41,7 @@ public:
     void dropPerformed() override;
     void dndFinished() override;
     void dndAction(DataDeviceManagerInterface::DnDAction action) override;
+    void dndCancelled() override;
 
     wl_resource *resource() const;
 

src/wayland/seat_interface.cpp

@@ -293,7 +293,7 @@ void SeatInterfacePrivate::endDrag(quint32 serial)
             dragTargetDevice->drop();
             dragSource->dropPerformed();
         } else {
-            dragSource->cancel();
+            dragSource->dndCancelled();
         }
     }