[KJots] Drop custom internal links, use akonadi default ones

Summary:
No need to maintain custom URL schemas, as akonadi already has its own, together with various
helper functions to work with it.

Remove UrlRole and use EntityUrlRole instead (use akonadi:?item=... links).
Remove custom internal_link clipboard property, just use the default mechanism for URL setting.
Teach KJotsEdit to insert akonadi links too. This also improves drag-n-drop from treeview,
which is now pasted properly (i.e. title of page/book instead of link itself)

Test Plan:

1. Bookmarks work, both pages and books
2. KJotsLinkDialog works nicely
3. Tooltips also works
4. Ctrl+Click navigation works

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D29162