experimental/editor: progress

  - selection
  - some state moved out of editor.
  - Editor::getPosition() translates x,y mouse position into text
    position
  - General Editor::move() function for moving cursor
  - Editor::insert() (does not yet handle newlines)
  - Editor::remove() (does not yet delete across lines)
  - new StringSlice class to replace SkString.

Change-Id: I1ca03247a745bc045e41619cd5a839c495dc405b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/211884
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
6 files changed