Drop KWindowSystem dependency

Summary:
KWindowSystem activateWindow does the same thing as the QPA does when
calling QWindow::requestActivate.

We can save a dependency.

Test Plan:
Windows:
Read code for windows QPA + kwindowsystem and it seemed like the
same underlying call

X11:
Ran ./ktextedittest. pressed control+f. Window had focus

Wayland:
Both calls will no-op. It needs new API, but when that does exist
I'll make sure it's in QWayland.

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12413

Differential Revision: https://phabricator.kde.org/D26159