[effects/slidingpopups] Overhaul slotPropertyNotify

Summary:

• Slightly improve readability;
• Check that offset and location have been passed;
• Sanitize slide in/out duration (if any of those is equal to 0, KWin will crash).

Test Plan:

• Launched Yakuake;
• Pressed F12 several times (Yakuake still slides in/out).

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D14862