Add "Open a Session" action for app jump list in task manager & Co.

Test Plan:
Opening the context menu of a running kdevelop instance in Plasma task
manager has an action "Open a Session" which when executed starts a new
kdevelop instance which present the session selection dialog first.

Reviewers: KDevelop, brauch

Reviewed By: KDevelop, brauch

Subscribers: brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9074