Given that we only render one page at a time, this will lead to only *one* page-loadingIcon being displayed at a time even if multiple pages are visible in the viewer. However, this will make it clearer which page is the currently parsing/rendering one.
To simplify toggling of the page-loadingIcon visibility, the existing `PDFPageView.renderingState` is changed into a getter/setter-pair with the latter also handling the page-loadingIcon state.
An additional benefit of these changes is that the `PDFViewer` no longer needs to handling toggling of page-loadingIcon visibility during rendering, since there can only ever be *one* page rendering.
Finally, this may also simplify future changes w.r.t. page-loadingIcon visibility toggling (using e.g. a show-timeout).
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