Add native window retrieval on Linux.

Functionality had to be moved to a separate source file due to namespace
pollution from X11 headers.

Note that this doesn't set proper parent of file dialogs on Linux, as
GTK is broken and requires passing a GtkWindow parent.

profiler/src/NativeWindow.cpp

@@ -0,0 +1,24 @@
+#include "NativeWindow.hpp"
+
+#include <GLFW/glfw3.h>
+
+#ifdef _WIN32
+#  define GLFW_EXPOSE_NATIVE_WIN32
+#  include <GLFW/glfw3native.h>
+#elif defined __linux__
+#  define GLFW_EXPOSE_NATIVE_X11
+#  include <GLFW/glfw3native.h>
+#endif
+
+extern GLFWwindow* s_glfwWindow;
+
+void* GetMainWindowNative()
+{
+#ifdef _WIN32
+    return (void*)glfwGetWin32Window( s_glfwWindow );
+#elif defined __linux__
+    return (void*)glfwGetX11Window( s_glfwWindow );
+#else
+    return nullptr;
+#endif
+}