Move X11 specific KGlobalAccel handling into the x11-standalone platform

Summary:
InputRedirection has a workaround to add a connect on a QAction which
is used for a global shortcut. This is specific to the X11 platform as
the xtime needs to be updated.

This change adds a new virtual method to the Platform and moves the
implementation into the X11 standalone platform. Thus it does no longer
gets called on Wayland.

Reviewers: KWin, Plasma

Subscribers: plasma-devel

Tags: KWin

Differential Revision: https://phabricator.kde.org/D4168