Make QPMCache a non-QObject

It seems the only use of the QObject-ness of QPMCache (backend for QPixmapCache) is to be able to run timer events.

This can be done in other ways, too (like an aggregated QTimer).

In the case of the backend for QDesktopServices::openUrl(), a similar change saved ~1.5KiB in text, ~0.3KiB in data size and 26 relocations: https://codereview.qt-project.org/c/qt/qtbase/+/395356