python: expose pluginmanager.AbstractManager.register_external_manager().

And *now* it's possible to use scene converter plugins.

doc/python/pages/changelog.rst

@@ -127,8 +127,9 @@ Changelog
 -   Exposed the :ref:`text` library
 -   Exposed the minimal interface of :ref:`utility.ConfigurationGroup` and
     :ref:`utility.Configuration`
--   Exposed :ref:`pluginmanager.AbstractManager.set_preferred_plugins()`, the
-    base :ref:`pluginmanager.AbstractPlugin` class and
+-   Exposed :ref:`pluginmanager.AbstractManager.set_preferred_plugins()`,
+    :ref:`pluginmanager.AbstractManager.register_external_manager()`, the base
+    :ref:`pluginmanager.AbstractPlugin` class and
     :ref:`pluginmanager.PluginMetadata`
 -   Fixed issues with an in-source build (see :gh:`mosra/magnum-bindings#13`)
 -   All CMake build options are now prefixed with ``MAGNUM_``. For backwards

src/python/corrade/pluginmanager.cpp

@@ -144,7 +144,8 @@ void pluginmanager(py::module_& m) {
             }
 
             return state;
-        }, "Unload a plugin", py::arg("plugin"));
+        }, "Unload a plugin", py::arg("plugin"))
+        .def("register_external_manager", &PluginManager::AbstractManager::registerExternalManager, "Register an external manager for resolving inter-manager dependencies", py::arg("manager"), py::keep_alive<1, 2>());
 
     py::class_<PluginManager::AbstractPlugin, PluginManager::PyPluginHolder<PluginManager::AbstractPlugin>>{m, "AbstractPlugin", "Base class for plugin interfaces"}
         /* Plugin interface string, search paths, suffix, metadata file suffix

src/python/magnum/test/test_trade.py

@@ -1047,6 +1047,19 @@ class Importer(unittest.TestCase):
         with self.assertRaises(KeyError):
             manager.set_preferred_plugins('ApngImporter', [])
 
+    def test_register_external_manager(self):
+        # This scenario is stupid in practice, but want to test it on the
+        # Importer API for consistency
+        converter_manager = trade.ImageConverterManager()
+        converter_manager_refcount = sys.getrefcount(converter_manager)
+
+        manager = trade.ImporterManager()
+        manager.register_external_manager(converter_manager)
+        self.assertEqual(sys.getrefcount(converter_manager), converter_manager_refcount + 1)
+
+        del manager
+        self.assertEqual(sys.getrefcount(converter_manager), converter_manager_refcount)
+
     def test_metadata(self):
         manager = trade.ImporterManager()
         manager.set_preferred_plugins('PngImporter', ['StbImageImporter'])