Add new session button to SessionsModel

Summary:
This (optionally) appends an entry to the SessionsModel to append a "New
Session" option. The lock screen / user switcher are then modified to
use it.

Messages.sh is modified so all components are covered by the same pot.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2594