Optimize QQuickTextEdit for larger documents.

The rationale is to not end up re-processing the whole document on each update
Since we know where the editing takes place, we can break down the text edit's
contents in several text nodes and only re-create the affected text nodes upon
editing.

This requires ripping out the SelectionEngine helper class from QQuickTextNode
so that QQuickTextEdit can tap into its functionality directly.

A positive side-effect of this exercise is that it should be much harder to
to come across GlyphNodes packing more than 16300 glyphs or so.

Task-number: QTBUG-29596
Change-Id: Id29b0709baa43f5b29c44ab02398ba996be3e28a
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>