Don't bind ShellClient::acceptsFocus to whether the window is shown

Summary:
A not shown window may accept focus (e.g. when minimized). Given that
the condition was wrong and broke when making minimized windows not
shown.

This change takes the idea of not passing focus to a closing or unmapped
window directly into acceptsFocus. Which also means that this condition
now works for xdg_shell windows.

BUG: 368673

Reviewers: KWin, Plasma on Wayland, bshah

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2745