support including a default_map

Dependencies

• [2] YOK66CEW small change to simplify the driver's task
• [*] LRDM35CE app running again
• [*] BJ5X5O4A let's prevent the text cursor from ever getting on a drawing
• [*] 36Z442IV back to commit 8123959e52f without code editing
• [*] 2CTN2IEF Merge lines.love

Change contents

• edit in live.lua at line 257
  [2.189]

  [4.7091]
  + elseif cmd == 'DEFAULT_MAP' then

  + local contents = love.filesystem.read('default_map')

  + if contents == nil then contents = '{}' end

  + live.send_to_driver(contents)
• edit in Manual_tests.md at line 12
  [6.8876]

  [7.94]
  + - default_map absent/present