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