Converting PDFFindBar and PDFFindController to classes

This PR:

- converts PDFFindBar and PDFFindController to classes;
- adjusts `viewer.js`, `page_view.js` and `text_layer_builder.js` accordingly;
- removes unnecessary globals and fixes minor code style issues.

Tip: https://github.com/mozilla/pdf.js/pull/5007/files?w=1 makes review much more pleasant.