[WIP] Converting PDFFindBar and PDFFindController to classes

This PR:

- converts PDFFindBar and PDFFindController to classes;
- adjusts `viewer.js` and `text_layer_builder.js` accordingly;
- removes unnecessary globals and fixes minor code style issues.

Although this seems to work as it should, I'm marking this as WIP until I get a chance to review and test it again tomorrow. Feel free to provide feedback in the meantime!

Tip: https://github.com/mozilla/pdf.js/pull/5007/files?w=1 makes review much more pleasant.
You could/should move this down and place it after `if (this.integratedFind) { ... }`, since it really doesn't if `this.findBar` is defined when the integrated find is used.