add ellipse tool

[jacekpoz]

the current behaviour is the middle of the circle stays where you started dragging

I could change it so that it functions similar to the rectangle but it's 1 in the morning and I can't really think

if you do want that I'll do it in the morning

[gabm]

looks good..

do you think you can add the shift modifier to draw circles (radii.x = radii.y)?

[jacekpoz]

I'll also add the same thing to the rectangle tool in another PR