Platform: add constructor for *Application::Arguments structs.

Apparently calling std::optional<Arguments>::emplace() doesn't work on a
struct that doesn't have any constructor.

src/Magnum/Platform/AbstractXApplication.h

@@ -62,6 +62,9 @@ class AbstractXApplication {
     public:
         /** @brief Application arguments */
         struct Arguments {
+            /** @brief Constructor */
+            /*implicit*/ constexpr Arguments(int& argc, char** argv) noexcept: argc{argc}, argv{argv} {}
+
             int& argc;      /**< @brief Argument count */
             char** argv;    /**< @brief Argument values */
         };

src/Magnum/Platform/GlutApplication.h

@@ -97,6 +97,9 @@ class GlutApplication {
     public:
         /** @brief Application arguments */
         struct Arguments {
+            /** @brief Constructor */
+            /*implicit*/ constexpr Arguments(int& argc, char** argv) noexcept: argc{argc}, argv{argv} {}
+
             int& argc;      /**< @brief Argument count */
             char** argv;    /**< @brief Argument values */
         };

src/Magnum/Platform/Sdl2Application.h

@@ -178,6 +178,9 @@ class Sdl2Application {
     public:
         /** @brief Application arguments */
         struct Arguments {
+            /** @brief Constructor */
+            /*implicit*/ constexpr Arguments(int& argc, char** argv) noexcept: argc{argc}, argv{argv} {}
+
             int& argc;      /**< @brief Argument count */
             char** argv;    /**< @brief Argument values */
         };

src/Magnum/Platform/WindowlessCglApplication.h

@@ -96,6 +96,9 @@ class WindowlessCglApplication {
     public:
         /** @brief Application arguments */
         struct Arguments {
+            /** @brief Constructor */
+            /*implicit*/ constexpr Arguments(int& argc, char** argv) noexcept: argc{argc}, argv{argv} {}
+
             int& argc;      /**< @brief Argument count */
             char** argv;    /**< @brief Argument values */
         };

src/Magnum/Platform/WindowlessGlxApplication.h

@@ -101,6 +101,9 @@ class WindowlessGlxApplication {
     public:
         /** @brief Application arguments */
         struct Arguments {
+            /** @brief Constructor */
+            /*implicit*/ constexpr Arguments(int& argc, char** argv) noexcept: argc{argc}, argv{argv} {}
+
             int& argc;      /**< @brief Argument count */
             char** argv;    /**< @brief Argument values */
         };

src/Magnum/Platform/WindowlessWglApplication.h

@@ -95,6 +95,9 @@ class WindowlessWglApplication {
     public:
         /** @brief Application arguments */
         struct Arguments {
+            /** @brief Constructor */
+            /*implicit*/ constexpr Arguments(int& argc, char** argv, HWND window) noexcept: argc{argc}, argv{argv}, window{window} {}
+
             int& argc;      /**< @brief Argument count */
             char** argv;    /**< @brief Argument values */
             #ifndef DOXYGEN_GENERATING_OUTPUT