add camera blocks (fix #460)

*Work in progress*
The stage gets two new pen blocks.
The camera picture is painted as pen trail, that means that the sprites are able to recognize colors with the sensing blocks but also that trails / stamps will be overwritten.
I *love* how you've done all of this wonderful work and integrated it so extremely well into Snap's codebase! Please be patient, as it will take me some more time to review all of your pull-requests (it's vacation time in Germany, and I'm just getting back from the Scratch Conference). So, don't be discouraged if I need more time (perhaps even a few weeks) and if it appears as though I'm not interested, because *I totally am very interested in this!*. Thanks!!