KSelectionOwner usage in ApplicationX11 and Compositor classes

Summary:
Declare it in the source file for internal usage of ApplicationX11 and
Compositor classes. Additionally use modern style connect and do other
code style updates.

Test Plan: Manually in X session and Wayland windowed.

Reviewers: KWin, zzag

Subscribers: davidedmundson, sbergeron, zzag, kwin

Tags: KWin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D21655