Add Meta+L shortcut for "Lock Session"

Summary:
This shortcut is used by default on Windows and it's also nice to have global shortcuts
use the Meta key in general.

Test Plan:

• Log into a new user account or reset the ksmserver section in the Shortcuts KCM
• Hit Meta+L or ctrl+alt+L
• Observe that both work

Reviewers: Plasma, VDG, broulik, GB_2

Reviewed By: VDG, GB_2

Subscribers: nicolasfella, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20178