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

Summary:
A common power user complaint about Plasma is the lack of a
{key 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; {key 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 {nav Shortcuts} KCM:
{F5950456}

Reviewers: davidedmundson, #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: #plasma, tcanabrava, konsole-devel

Tags: #konsole

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