python: make it possible to specify application Configuration as kwargs.

So it can be done in a single expression, like in C++. Originally I
thought I'd make the Application constructor itself take kwargs,
combined from Configuration and GLConfiguration, but this would be
annoying to implement and annoying to use because it wouldn't be
possible to perform the usual pattern of try_create(), fail, set
different parameter, create().

src/python/magnum/platform/application.h

@@ -38,7 +38,19 @@ namespace magnum { namespace platform {
 
 template<class T> void configuration(py::class_<T>& c) {
     c
-        .def(py::init())
+        .def(py::init([](const std::string& title, const Vector2i& size, typename T::WindowFlag windowFlags) {
+            return T{}
+                .setTitle(title)
+                .setSize(size)
+                .setWindowFlags(windowFlags);
+        }), "Constructor",
+            #if PYBIND11_VERSION_MAJOR*100 + PYBIND11_VERSION_MINOR >= 206
+            py::kw_only{}, /* new in pybind11 2.6 */
+            #endif
+            /** @todo drop std::string in favor of our own string caster */
+            py::arg("title") = std::string{T{}.title()},
+            py::arg("size") = T{}.size(),
+            py::arg("window_flags") = typename T::WindowFlag(typename std::underlying_type<typename T::Configuration::WindowFlag>::type(T{}.windowFlags())))
         .def_property("title",
             /** @todo drop std::string in favor of our own string caster */
             [](T& self) -> std::string {
@@ -64,7 +76,7 @@ template<class T> void configuration(py::class_<T>& c) {
 template<class T, class Trampoline, class Holder> void application(py::class_<T, Trampoline, Holder>& c) {
     py::class_<typename T::GLConfiguration> glConfiguration{c, "GLConfiguration", "OpenGL context configuration"};
     glConfiguration
-        .def(py::init());
+        .def(py::init(), "Constructor");
         /** @todo others */
 
     c