[KJots] Fix bookmarks actions

Summary:
This patch ports KJots from deprecated `KBookmarksMenu` API (see {D25660}).

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