[wayland] Don't honor the idle inhibitor object if the surface is not visible

Summary:
Currently, our implementation of zwp_idle_inhibitor_v1 is not fully
spec-compliant. If the associated surface is not visible, we still
honor the idle inhibitor object.

This change addresses those spec-compliance issues. If a surface is
minimized or it's not on the current virtual desktop, then the
associated inhibitor object won't be honored by KWin.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: graesslin, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17343