Properly implement maximize of ShellClient

Summary:
This brings some more checks from Client to ShellClient. Thus the
states are better adjusted.

Unfortunately the X11 implementation is also slightly adjusted, so could
create regressions in worst case.

BUG: 368393

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3507