[KJots] Add support for headings
Summary:
This patch adds support for headings, introduced for underlying KRichTextEdit in D28854: [KRichTextWidget] Add support for headings.
It adds relevant actions to toolbars & menu.
This also increases the required KF5 version to (not yet released) 5.70.
Slighly overlaps with D29291: [KJots] Provide explicit "Save" action, use standard actions where possible (which increases the version in .rc-file)
CCBUG: 230317
Test Plan: it compiles, actions are added (and functional)
Reviewers: dvratil, mlaurent
Subscribers: mlaurent, kde-pim
Differential Revision: https://phabricator.kde.org/D29300