Start working on a GUI kernel API.

Kernel/GUITypes.h

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+#pragma once
+
+// GUI system call API types.
+
+struct GUI_Rect {
+    int x;
+    int y;
+    int width;
+    int height;
+};
+
+struct GUI_WindowFlags { enum {
+    Visible = 1 << 0,
+}; };
+
+typedef unsigned GUI_Color;
+
+struct GUI_CreateWindowParameters {
+    GUI_Rect rect;
+    GUI_Color background_color;
+    unsigned flags;
+    char title[128];
+};