[KJots] Fix bookmarks actions

Summary:
This patch ports KJots from deprecated KBookmarksMenu API (see D25660: Decouple KBookmarksMenu from KActionCollection).

On my machine, it also fixes a crash (bm_action was null for some reason,
I did not investigate it though).

It also fixes the shortcuts collision (action->setShortcut was not enough,
one also needs to run actionCollection->setDefaultShortcut).

BUG: 384000

Test Plan:

1. Start KJots
2. It don't crash, and Ambiguous shortcuts dialog does not appear)
3. Open Bookmarks menu, the shortcut is Ctrl+Shift+B.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: winterz, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28775