BuildSystem: Cleanup userspace directory layout

userspace programs are now in userspace/programs
userspace tests are now in userspace/tests

This makes listing userspace projects much cleaner. Libraries were
already separated to their own directory, so other programs should also.

userspace/programs/WindowServer/Framebuffer.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "Utils.h"
+
+struct Framebuffer
+{
+	int fd;
+	uint32_t* mmap;
+	int32_t width;
+	int32_t height;
+	uint8_t bpp;
+
+	Rectangle area() const { return { 0, 0, width, height }; }
+};
+
+Framebuffer open_framebuffer();