Clear both cursor caches

Summary: There are two cursor caches: m_cursors and m_cursorsByName. The second doesn't ever seem to be cleared.

Reviewers: #kwin, davidedmundson, graesslin

Reviewed By: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D12542