Block layout nitpicks

[hildjj]

This input:

block Do a thing
  A->B: Some messages
end

loop Many times
  A->B: Some messages
end

opt Might happen
  A->B: Some messages
end

yields this output:

Nits:

• the block text should be moved horizontally so it starts to the right of the A line.
• the opt and loop tabs are a little too short. Those should be driven by the font size, plus a margin
• the baselines for the text "opt" and the text "Might happen" should either be aligned, or be a little more different, so that it doesn't just look like sloppiness. Same for loop.

@ekr would you mind taking a look at these since you were just in there?

[ekr]

Yeah, I can take a look. We can, as you suggest estimate based on the font size, but it seems like it would be better if draw_label() actually returned the width. It seems like at least PDFDriver.js knows this information, but I'm not sure if it's in the right units.

[hildjj]

Calculating font metrics correctly outside the browser for SVG is going to be a PITA, since I don't want to take a dependency on canvas or similar. It's fine to just put a quick heuristic in.