Make support for the minimized state optional

Summary:
In long term, we want to split XdgShellClient into several classes. One
class for xdg-toplevel clients, and the other one for xdg-popup clients.

xdg-popup clients are much simpler than xdg-toplevel clients, they can't
be maximized or shown in full screen mode, they can't be interactively
moved on the screen, and so on. In the end, we will have to plumb many
pure virtual methods, which looks a bit ugly.

This change makes support for the minimized state in AbstractClient
optional so we don't have to add those no-op methods and keep code more
or less "clean."

Test Plan: Compiles.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27163