tune editing actions for large number of small edits

Summary:
example workload that triggers these bottlenecks: mass replace of a character in a large
document

see: https://phabricator.kde.org/D17437

bug report with test case file: https://bugs.kde.org/show_bug.cgi?id=333517

these changes are not sufficient to have it working properly but perf already tells it is much better

Reviewers: dhaumann, Kate, loh.tar

Subscribers: anthonyfieroni, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D17441