Sync QWindow flag WindowDoesNotAcceptFocus to wayland plasmashell interface

Summary:
Currently the widget explorer relied on some QML code calling
KWindowSystem::forceActiveWindow which on plasma implicitly set this
hint.

This works for the first show, but the order of events is somewhat
unreliable. It's invoked from QML when the window is ready, but for the
second show this is before the wl_surface is created and it no-ops.

By syncing the flags in applyType() we can guarantee we call this both
when setFlags is called and when the wl_surface is created in the
correct order.

BUG: 415124
BUG: 401172

Test Plan:
Ran plasmashell
Opened widget explorer twice
Had focus each time

Reviewers: KWin, Plasma, romangg

Reviewed By: KWin, Plasma, romangg

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26153