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.

(cherry picked from commit 41b2b35)

src/model/interface.h

@@ -20,6 +20,8 @@
 #ifndef INTERFACE_H
 #define INTERFACE_H
 
+#include <QMetaObject>
+
 #include <functional>
 
 /**
@@ -48,15 +50,15 @@
  */
 #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
  * @brief Decalre signal-like method. Should be used in interface
  */
 #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); \
     }