Revert "Remove modules/skplaintexteditor"

This reverts commit 945284d57785315ea6087518afc79b153d2476ab.

Reason for revert: This is still useful as a Viewer-like interface for SkShaper

Original change's description:
> Remove modules/skplaintexteditor
>
> It has been unused for 3+ years.
>
> Change-Id: Ie084cb10a7b9453771bddf07d9c1eaeac3b5558a
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/772899
> Reviewed-by: Brian Osman <brianosman@google.com>

Change-Id: Ieb8a092a9b723efdd48fce6840a62b58da4fd8d1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/773816
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
13 files changed