UI: Move scene import dialog to the stack

Changed for consistency as almost every other dialog in the application
is stack-allocated.

UI/window-basic-main-scene-collections.cpp

@@ -367,10 +367,8 @@ void OBSBasic::on_actionRemoveSceneCollection_triggered()
 
 void OBSBasic::on_actionImportSceneCollection_triggered()
 {
-	OBSImporter *imp;
-	imp = new OBSImporter(this);
-	imp->exec();
-	delete imp;
+	OBSImporter imp(this);
+	imp.exec();
 
 	RefreshSceneCollections();
 }