derive size of text-containing widgets from size of their text

[cmyr]

Widgets like button and textbox currently have different methods for determining their size. Button defers to label, which is sized based on the contained text; and textbox uses a constant defined in the env.

Basically: any widget that contains text should have have its size based on the size of the underlying text, so that the widgets will resize correctly in response to changes in text size. This is important not just to make these widgets more readily customizable, but also to support things like adaptive text size, an important accessibility feature.

This is currently blocked on linebender/piet#134, which will give us the ability to measure the height of lines.

[cmyr]

Fixed as part of #1182