Use reuseItems property on listviews, and unload cover images when delegates are not used

Set reuseItems to true on ListViews, which pools delegates and reuses them when they are loaded.

To reduce memory usage, unload images when the delegate is not being used.

Note: Qt version was bumped to 5.15 in order to use the feature.

https://doc.qt.io/qt-5/qml-qtquick-tableview.html#reusing-items