core/reloader: trigger postReload with a signal

A signal is now used over the previous tree-searching method as some
QML components such as Repeater fail to reparent created children to
themselves, which breaks the tree.

src/core/singleton.cpp

@@ -51,9 +51,3 @@ void SingletonRegistry::onReload(SingletonRegistry* old) {
 		singleton->reload(old == nullptr ? nullptr : old->registry.value(url));
 	}
 }
-
-void SingletonRegistry::onPostReload() {
-	for (auto* singleton: this->registry.values()) {
-		PostReloadHook::postReloadTree(singleton);
-	}
-}