Only trigger mod-only-shortcuts if global shortcuts are enabled

Summary:
KWin supports blocking global shortcuts when a window is active through
window specific rules. This change ensures that the modifier only
shortcuts also honor the blocking of global shortcuts. If global
shortcuts are disabled, they won't trigger any more.

BUG: 370146
FIXED-IN: 5.8.1

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D2952