Summary:
Not all Client classes have support for the maximized mode. Therefore,
it can be made opt-in to reduce the amount of plumbed methods.

Unfortunately, there a few places, which don't have any connection with
the maximized mode, where AbstractClient::geometryRestore() is used, so
it cannot be made 100% optional at the moment.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27169