bugfix: recompute screen lines in backspace/delete

Dependencies

• [2] 2RXZ3PGO beginning of a new approach to scroll+wrap
• [*] BULPIBEG beginnings of a module for the text editor

Change contents

• edit in text.lua at line 206
  [2.2359]

  [2.2359]
  + Lines[Cursor1.line].screen_line_starting_pos = nil
• edit in text.lua at line 218
  [2.2725]

  [2.2725]
  + Lines[Cursor1.line-1].screen_line_starting_pos = nil
• edit in text.lua at line 254
  [2.3296]

  [2.3296]
  + Lines[Cursor1.line].screen_line_starting_pos = nil
• edit in text.lua at line 264
  [2.3602]

  [2.3602]
  + Lines[Cursor1.line].screen_line_starting_pos = nil