LibGfx: Add Gfx::Path, a basic 2D path with <canvas> semantics

This will be used to implement painting of 2D paths. This first patch
adds support for line_to(), move_to() and close().

It will try to have the same semantics as the HTML <canvas> element.

To stroke a Path, simply pass it to Painter::stroke_path().

Libraries/LibGfx/Forward.h

@@ -42,6 +42,7 @@ class ImageDecoder;
 class Painter;
 class Palette;
 class PaletteImpl;
+class Path;
 class Point;
 class Rect;
 class ShareableBitmap;