[lively-kernel] Text margin

[Daniel Ingalls]

Folks -

I have added an inset ("padding") of (4, 2) to the prototypical text in the parts bin, as well as the documentation windows and the workspace part.

This makes text look nicer and makes it easier to select near the edge, but it could cause problems with pre-existing objects if they were carefully laid out.  My apologies in advance if this happens -- I'm willing to help fix them.

  - Dan