Extend BoxModel to collapse framing as requested

[japhb]

(Split out from #11 because there's a lot to think about here.)

• Each level of the BoxModel framing should be collapsible in sequence if requested
• Inner framing layers can only collapse if outer layers collapsed completely to zero along that edge
• Must be able to set different collapse rules for each side
• What happens when the user clicks on a collapsed frame?
• Will need rules for collapsing different framing styles
  • Which color wins for colored margin or padding collapse?
  • Which border style wins for overlaid borders?
  • What if one or both borders are active/dynamic (have animations or embedded text/styling)?
  • How should different styles be handled at T-intersections, +-intersections, and corners?
  • What about framing that changes depending on state, such as switching style when focused?

[patrickbkr]

There even exist double border chars (I surely haven't picked all the right ones from https://en.wikipedia.org/wiki/Box-drawing_character, but the idea should be clear):

┌─╥─┐ 
│ ║ │   
└─╨─┘

Also it should be possible to define collapsing rules for the screen border (i.e. no border at the screen edge).