Review IdleRegister (#4892)

This turns `IdleRegister::add()` into a template function to make the
provided function pointers type safe. It also adds a `delete_data`
parameter to manage the provided data pointer even in `destroy()`.

rtgui/xtransprocess.cc

@@ -255,18 +255,19 @@ void XTransProcess::checkBoxToggled (CheckBox* c, CheckValue newval)
 void XTransProcess::autoContrastChanged (double autoContrast)
 {
     struct Data {
-        XTransProcess *me;
+        XTransProcess* self;
         double autoContrast;
     };
-    const auto func = [](gpointer data) -> gboolean {
-        Data *d = static_cast<Data *>(data);
-        XTransProcess *me = d->me;
-        me->disableListener();
-        me->dualDemosaicContrast->setValue(d->autoContrast);
-        me->enableListener();
-        delete d;
-        return FALSE;
-    };
 
-    idle_register.add(func, new Data { this, autoContrast });
+    const auto func =
+        [](Data* data) -> bool
+        {
+            XTransProcess* self = data->self;
+            self->disableListener();
+            self->dualDemosaicContrast->setValue(data->autoContrast);
+            self->enableListener();
+            return false;
+        };
+
+    idle_register.add<Data>(func, new Data{this, autoContrast}, true);
 }