Emit pageChange on MultiPageViewer

[jain-amit]

This change mirrors the change done for SInglePageViewer

Verified works locally on scrolling, find, zoom and other functionality.

Fixes
Issue #223

[VadimDez]

Thank you for the PR!
I've just tried the code, but it seems like the event is not fired. Did you change anything else in the code?

[jain-amit]

Thanks for the quick response.

I didn't change anything else.

I struggled with making it work until I got the scrollbar to appear.

To get the scrollbar to appear I added display to use flexbox and height that is calculated to fit into the viewable area.

The flexbox in the viewable area lets a toolbar be at the top without moving when scrolling a pdf

Getting the scrollbar was the key to get the event to emit.
This also made the currentPageNumber have the correct value.

This is an example:

    <pdf-viewer
      style="display: flex; height: 80vh"
      [src]="'url'"
      [original-size]="false"
      [render-text]="true"
      [render-text-mode]="RenderTextMode.ENHANCED"
      [show-all]="true"
      (after-load-complete)="afterLoadComplete($event)"
      (pageChange)="pageChanging($event)"
    >
    </pdf-viewer>

[jain-amit]

Also adding the scrollbar made it so that not all the PDF was rendered immediately for MutliPageViewer. This makes it possible to view very large PDFs on devices with a small amount of memory. There is a rolling window of pages that are rendered.