Scroll to page or to annotation

[lfoppiano]

This PR proposes a simple method to scroll down to a specific page or an annotation. Since Streamlit has quite a complex approach which reload at each interaction, it makes hard for any component to handle the scrolling in a customized way.

With this PR we added two additional parameters: scroll_to_page and scroll_to_annotation (mutually exclusive, scrol_to_page has priority) which require an integer in input. The integer represent the positional item (page or annotation) from 1 to n.

In order to know in advance number of pages and annotations, the pdf_viewer(...) call returns a dict as follow:

{ 
'pages: [
   'id1', 
   'id2'
  ], 
'annotations': [
   'ann-id1', 
   'ann-id2'
  ....
]
}

Those ids are the same used in the DOM and may be used for javascript calls, however I did not manage to achieve anything with it. Nevertheless, they can be used to build a set of buttons that allow calling again the pdf_viewer with proper parameters for pages or annotations.

This PR should solve issue #24, #47, #55, and #61