Port KNotification from KWindowSystem

Summary:
KWindowSystem::activateWindow does the same system call as
QWindow::requestActivate for X11 and Windows and both currently do
nothing on wayland.

May as well use Qt implementation

Reviewers: nicolasfella

Reviewed By: nicolasfella

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26513