Moves watchScroll and getVisibleElements from PDFView
Moves thumbs logic into PDFThumbnailViewer.
Moves viewer code into PDFViewer and some code from PageView.
Moves pdfDocument.getPage/getTextContent requests out of PDFView
Moves rendering queue logic from PDFView
Removes PresentationMode dependency from PDFViewer
Removes any usage of PDFView in the PageView
Moves constants to avoid dependency on PDFView
Marks some private methods in PDFViewer and PDFThumbnailViewer

Conflicts:
	web/pdf_viewer.js
Adds types definitions (jsdoc) for the PDFViewer code.
Renames and refactors PDFView to PDFViewerApplication.
Merge pull request #5295 from yurydelendik/pdfviewer

Refactoring to move page display code into separate class