Call Platform::setupActionForGlobalAccel on the Client shortcut

Summary:
Platform::setupActionForGlobalAccel is an important call on X11 platform.
Without the x11 timestamp doesn't get updated and calls in KWin might
fail - e.g. the activation of the Client which is supposed to happen.

Test Plan: compiles

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D6802