qt: get rid of `RoundImage` and re-introduce it with a type that directly derives from `QQuickImage`

We really need to unify the way images are shown in the UI.

We really need to consider using MSAA, as I pointed out several times before, since we exceed the render batches that Qt recommends considerably. I observed 1 opaque batch, and up to 50 translucent batches. Qt's recommendation is 3-4 opaque batches, and less than 10 translucent batches [1].

If we can use MSAA, we can go this way. And the next step would be to use smaller size texture in grid items so that they are placed in the atlas and delegate is batch rendered.

[1] https://doc.qt.io/qt-6/qtquick-visualcanvas-scenegraph-renderer.html#performance

Request review @chub.