[KRichTextEdit] Always treat key press as single modification in undo stack

Summary:
If my cursor is on a bullet list, and I press Enter to create a new element,
it registers as 5-7 actions in an undo stack, mostly margins adjustments. So to
rewind it, user has to press Ctrl+Z 5+ times, which is quite frustrating.

Instead this patch suggests to treat _any_ change that is a result of a single
key press as a single event, so user has to press Ctrl+Z just once to undo it

BUG: 256001

Test Plan:

1. Open KMail -> New Mail, enable Rich Text mode (or, say, open KJots)
2. Create a bullet list with couple of elements
3. Put cursor on the last element, press Enter to create a new element
4. Try to undo this action with Ctrl+Z
5. (without patch) One has to press Ctrl+Z 5-7 times to undo it.
6. (with patch) A single Ctrl+Z is enough

Reviewers: Frameworks, mlaurent, dfaure

Reviewed By: dfaure

Subscribers: ahmadsamir, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28819