When calling setDocument (directly or through the constructor) a delayed
rehighlight is initiated. Previously, if any text was changed before
this rehighlight could run it would cancel the rehighlight, even if the
changed text only caused a new block of text to be highlighted.
Fixes: QTBUG-71307
Change-Id: Ib09b664d90906f5b4427105f0e45469806f3a779
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>