TextEdit: Better support of QTextTable and inline images

Fix some issues with incremental updates found while
playing with the Quick Controls textedit demo.

 -Grouping text blocks into a single text node is fine as
long as it doesn't cross the boundary of a child frame
(e.g. a table), otherwise all that node logic collapses.

 -Text tables are hard to split in a sensible way, ensure
we treat them as one text node for the sake of simplicity.

 -Inline images can cause several text nodes to have the
same apparent start position. Beef up the rewinding logic
in markDirtyNodesForRange.

Change-Id: Ib4518bcd9303035fa00d9f4b16da7ca6c88e2313
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>