Added "colour_random" block to generate a random colour.

Review URL: https://codereview.appspot.com/8540044

git-svn-id: http://blockly.googlecode.com/svn/trunk@749 c400ca83-b69d-9dd7-9705-49c6b8615e23
Factor out common text from dropdown menus.
Review URL: https://codereview.appspot.com/8748043

git-svn-id: http://blockly.googlecode.com/svn/trunk@750 c400ca83-b69d-9dd7-9705-49c6b8615e23