Add a global shortcut (Ctrl+Alt+T) to launch Konsole

Summary:
A common power user complaint about Plasma is the lack of a
Ctrl+Alt+T keyboard shortcut to open Konsole. It turns out
that one has existed for quite some time in the form of an
off-by-default example for instructional purposes, shipped with
khotkeys.

This patch adds one such shortcut to Konsole itself, ensuring that
it will always be available when Konsole is installed.

Test Plan:
Either log into a new user account or reboot into an existing user
account; Ctrl+Alt+T opens Konsole! It even includes the
bouncing app launch animation since Konsole opened via the
desktop file.

The shortcut itself shows up as a sub-item under the KMenuEdit
category in the Shortcuts KCM:

Reviewers: davidedmundson, Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: Plasma, tcanabrava, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D13708