Create a global launch shortcut (Meta+E)

Summary:
This patch adds a global shortcut ({key Meta E}) to launch Dolphin, just like Konsole has.

FEATURE: 405302
FIXED-IN: 19.04.0

Test Plan:
1. Apply patch and reboot, or create and log into a new user
2. Hit {key Meta E}
3. Dolphin opens!

Reviewers: #dolphin, #vdg, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: hallas, elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19831