Display shortcut badges while holding Super

Summary:
Use `KModifierKeyInfo` to display shortcut badges while holding Super

BUG: 401768

Test Plan:
Just playing with it.
I still don't understand the `GlobalShortcut` class very well, correct me if I make any mistake.

Reviewers: #latte_dock, mvourlakos

Reviewed By: #latte_dock, mvourlakos

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18620