add a trash can to turtle art

[walter]

kids don't seem to connect to the idea of returning bricks to the palette and when they do, more often than not, they drag the bricks past the palette off the left edge of the screen. Either restrict the bricks from going past the left edge and/or add a separate trashcan.

[RafaelOrtiz]

this have proven to be right on etoys, so it's a good suggestion to have a trash can also in TA.

[walter]

As a temporary measure, I'm preventing blocks from going off the left-hand edge of the screen, so if they do get put back on the palette, they will more consistently be deleted.

[walter]

There is a trash palette now... let's see how well that works.