Add white space rendering to Text fields

[jdunkerley]

Want to use a fixed width font. It should not do glyphs (i.e. render => as two character no a merged glyph)

We should render whitespace in the normal way.

• Space => .
• Tab => ``
• Return => ``
• New Line => `` (and a new line in the cell).

They should be in a light grey.

Button to switch on the rendering on/off/special (default)

• special means leading/trailing whitespace is rendered.
• multiple spaces and non-space chars rendered.
• \r should be rendered.
• We should ensure \n actually causes a new line.