Fix interaction between decoration and fullscreen ShellClient

Summary:
A fullscreen ShellClient still had a decoration which was caused by
a combination of several bugs:

• when going to/from fullscreen the decoration was not update
• noBorder did not return true for a fullscreen window
• wl_shell emits fullscreen changed and maximized changed

Comparing to X11 Client both is done. So ShellClient needs to do the
same. This ensures that the correct geometry is requested when going
to fullscreen.

BUG: 366764

Reviewers: KWin, Plasma on Wayland, bshah

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2751