Annotations: Better page navigation

[Ostrzyciel]

The current solution for loading pages in the annotation module requires the annotator to manually click on buttons with page numbers at the bottom of the page, which is sufficient, but not optimal. Page navigation can be further simplified by implementing a dynamic page loading mechanism in which a few pages would be loaded in advance, responding to the user scrolling the document. Pages far away from the user’s view would be unloaded from memory to save system resources. This would mimic the behaviour of many modern social media applications that allow scrolling their content feed almost infinitely.