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/mydiagonalcurve.cc

@@ -1516,28 +1516,28 @@ void MyDiagonalCurve::updateBackgroundHistogram (LUTu & hist)
 
     mcih->pending++;
 
-    const auto func = [](gpointer data) -> gboolean {
-        MyCurveIdleHelper* const mcih = static_cast<MyCurveIdleHelper*>(data);
+    const auto func =
+        [](MyCurveIdleHelper* mcih) -> bool
+        {
+            if (mcih->destroyed) {
+                if (mcih->pending == 1) {
+                    delete mcih;
+                } else {
+                    mcih->pending--;
+                }
 
-        if (mcih->destroyed) {
-            if (mcih->pending == 1) {
-                delete mcih;
-            } else {
-                mcih->pending--;
+                return false;
             }
 
-            return 0;
-        }
+            mcih->clearPixmap ();
+            mcih->myCurve->queue_draw ();
 
-        mcih->clearPixmap ();
-        mcih->myCurve->queue_draw ();
+            mcih->pending--;
 
-        mcih->pending--;
+            return false;
+        };
 
-        return FALSE;
-    };
-
-    idle_register.add(func, mcih);
+    idle_register.add<MyCurveIdleHelper>(func, mcih, false);
 }
 
 void MyDiagonalCurve::reset(const std::vector<double> &resetCurve, double identityValue)