Revert model to use simple UTF-16 code units

This is a prerequisite to browser-based grapheme cluster handling, which
is needed so left/right cursoring and backspace behave as users expect.

modules/ve/ve.js
modules/ve/ce/ve.ce.Document.js
modules/ve/ce/ve.ce.js
* Revert cluster-aware splitting to trivial javascript code unit splitting
* Rewrite ve.splitClusters as a trivial compatibility method (remove soon)
* getClusterOffset/getByteOffset use unicodeJS.graphemebreak.splitClusters

modules/unicodejs/tools/unicodejs-properties.py
modules/unicodejs/unicodejs.graphemebreakproperties.js
modules/unicodejs/unicodejs.js
* Allow grapheme break tests to work with surrogate pairs

demos/ve/pages/minimal.html
demos/ve/pages/multibyte.html
demos/ve/pages/unicode.html
* replace file with more precise tests

modules/ve/test/ve.test.js
* Remove reference to grapheme-based splitting (which is no longer used)
* Correct typo

Bug: 53757
Bug: 51472
Bug: 51596
Bug: 51846
Change-Id: Ife34c87ebe40bc1689298b592eec5c0cdc2f7589
Merge "Revert model to use simple UTF-16 code units"