fix(model): return connection from interface macro

connection is normally returned from Qt's connect, and the caller may want to
track the connection to manually disconnect it.

src/model/interface.h

@@ -20,6 +20,8 @@
 #ifndef INTERFACE_H
 #define INTERFACE_H
 
+#include <QMetaObject>
+
 #include <functional>
 
 /**
@@ -48,7 +50,7 @@
  */
 #define DECLARE_SIGNAL(name, ...) \
     using Slot_##name = std::function<void (__VA_ARGS__)>; \
-    virtual void connectTo_##name(Slot_##name slot) const = 0
+    virtual QMetaObject::Connection connectTo_##name(Slot_##name slot) const = 0
 
 /**
  * @def DECLARE_SIGNAL
@@ -56,7 +58,7 @@
  */
 #define DECLARE_SIGNAL(name, ...) \
     using Slot_##name = std::function<void (__VA_ARGS__)>; \
-    virtual void connectTo_##name(Slot_##name slot) const = 0
+    virtual QMetaObject::Connection connectTo_##name(Slot_##name slot) const = 0
 
 /**
  * @def SIGNAL_IMPL
@@ -65,7 +67,7 @@
 #define SIGNAL_IMPL(classname, name, ...) \
     using Slot_##name = std::function<void (__VA_ARGS__)>; \
     Q_SIGNAL void name(__VA_ARGS__); \
-    void connectTo_##name(Slot_##name slot) const override { \
+    QMetaObject::Connection connectTo_##name(Slot_##name slot) const override { \
         connect(this, &classname::name, slot); \
     }