Implement the setDocument method for the find controller

Now it follows the same pattern as e.g., the document properties
component, which allows us to have one instance of the find controller
and set a new document to search upon switching documents.

Moreover, this allows us to get rid of the dependency on `pdfViewer` in
order to fetch the text content for a page. This is working towards
getting rid of the `pdfViewer` dependency upon initializing the
component entirely in future commits.

Finally, we make the `reset` method private since it's not supposed to
be used from the outside anymore now that `setDocument` takes care of
this, similar to other components.

examples/components/simpleviewer.js

@@ -55,7 +55,7 @@ container.addEventListener('pagesinit', function () {
   pdfViewer.currentScaleValue = 'page-width';
 
   if (SEARCH_FOR) { // We can try search for things
-    pdfFindController.executeCommand('find', {query: SEARCH_FOR});
+    pdfFindController.executeCommand('find', { query: SEARCH_FOR, });
   }
 });
 
@@ -70,4 +70,5 @@ pdfjsLib.getDocument({
   pdfViewer.setDocument(pdfDocument);
 
   pdfLinkService.setDocument(pdfDocument, null);
+  pdfFindController.setDocument(pdfDocument);
 });