Use less Toplevel::isClient()

Summary:
Prefer qobject_cast<> over Toplevel::isClient() because it's more type
safer and makes code a bit more readable.

Hopefully, one day we will be able to get rid of isClient() altogether.

Test Plan: Compiles.

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D26541