Move implementation of iconGeometry from ShellClient to AbstractClient

Summary:
This allows Client to use the Wayland-specific implementation if there
is no icon geometry set through the X11 way. That way Xwayland windows
have an icon geometry even if Plasma is using Wayland and setting the
icon geometry in the Wayland way. Which is expected as Plasma is
ignorant about the windowing system a PlasmaWindow uses.

In order to move the code from ShellClient to AbstractClient
WaylandServer gained a new findAbstractClient(Surface*) method which
is just like findClient(Surface*) with the difference that it returns
an AbstractClient instead of a ShellClient*.

Test Plan:
minimized/unminimized an X client on Wayland, verified
animation is correct (though broken in general for minimize)

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2530