exclude left margin from my word-split heuristic

Dependencies

• [2] UPCIYZEU drop an unnecessary level of indent
• [3] UHB4GARJ left/right margin -> left/right coordinates
• [4] JLU2RMC4 allow Text.nearest_pos_less_than to return 0
• [5] DGK5BPVI bugfix: UTF-8 in compute_fragments
• [6] GL4Q5WCV keep text from overflowing right margin
• [7] LF7BWEG4 group all editor globals
• [8] LNUHQOGH start passing in Editor_state explicitly
• [9] LXTTOB33 extract a couple of files
• [10] HGC5RGJP switch to line index in a function
• [*] BULPIBEG beginnings of a module for the text editor

Change contents

• replacement in text_tests.lua at line 435
  [3.9990]→[3.398:488](∅→∅)
  − App.screen.check(y, 'am ', 'F - test_draw_wrapping_text_containing_non_ascii/screen:2')
  [3.9990]

  [3.9991]
  + App.screen.check(y, 'am I', 'F - test_draw_wrapping_text_containing_non_ascii/screen:2')
• replacement in text_tests.lua at line 437
  [3.10026]→[3.489:582](∅→∅)
  − App.screen.check(y, 'I’m ', 'F - test_draw_wrapping_text_containing_non_ascii/screen:3')
  [3.10026]

  [3.2452]
  + App.screen.check(y, '’m a', 'F - test_draw_wrapping_text_containing_non_ascii/screen:3')
• replacement in text.lua at line 101
  [2.169]→[2.169:203](∅→∅)
  − if x < 0.8*State.right then
  [2.169]

  [2.203]
  + if (x-State.left) < 0.8 * (State.right-State.left) then