Design Mode: Ability to create and edit basic shapes

Carve should support a way to create and edit basic shapes geometry: <rect>, <ellipse>, <circle>, <line>.

This feature shall include a basic toolbar that has a Rect, Ellipse, Line tool that puts Carve in 'Shape Insert' mode. Click-dragging in the Design canvas creates a black filled/stroked shape. For instance, when the Rect tool is selected and the user drags from (100,80) to (150,20) then a rect is created with: x=100, y=20, width=50, height=60

This feature shall include 'grippies' that help highlight the select items as well as provide basic sizing editing capabilities in the four corners of the shape's bounding box. Hovering over grips changes the cursor to a 'resize' cursor.

If the top-left 'grippie' is dragged, it affects the shapes top-left point as well as its size. i.e. dragging a rect's top-left grip by (-20,-10) shall adust the rect's bounding box by: x -= 20, y-= 10, width += 20, height += 10.