todo:

  ☐ sync scroll @needsUpdate(because of images)
  ☐ regive focus to the right markdown view
  ☐ set the title of the preview
  ☐ preview.set_scratch(True)
  ☐ fix #4
  ☐ bug when empty `src`
  ☐ auto refresh preview if loading images
  ☐ call settings listener on_new too
  ☐ preview.wordWrap => True
  ☐ try/except for 404
  ☐ support hanchor (TOC) @big


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Archive:
 ✔ clean the code (syntax) @done (17-01-02 16:27) @project(todo)
 ✔ add 404 image @done (17-01-02 16:27) @project(todo)
 ✔ load images from internet (`https:`) @done (17-01-02 15:51) @project(todo)