Added clear option

[DipanshKhandelwal]

issue #14 .
This will clear canvas.

[walterbender]

Two issues:
(1) I would have thought the clear would just cleat the drawing, not the dots.
(2) the clear button itself should be on the left, not next to the stop button. Plus it is not rendering properly for me.

[DipanshKhandelwal]

@walterbender issue (2) solved ,it will render properly and now its on the left side of tool bar.

[walterbender]

Black on dark gray is hard to read. Maybe you can grab a trash icon from another project, e.g., https://github.com/sugarlabs/physics/blob/master/icons/clear_all.svg

[DipanshKhandelwal]

Ok thanks 👍

[DipanshKhandelwal]

@walterbender take a look at the changes once.

[walterbender]

Everything is much improved, except that the clear button should not erase the current set of dots, just the drawing. (or it should restore the current set of dots after clearing)