Add --view option to display incidence

Summary:
Similar to kmail --view, this opens only the incidence viewer with the specified
incidence. Also introduces korganizer-view service to allow opening the URL
through a desktop file.

Test Plan: korganizer --view "akonadi://?item=<some-incidence-item-akonadi-id>"

Reviewers: #kde_pim, winterz

Reviewed By: #kde_pim, winterz

Subscribers: winterz, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29530