[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 globals that became unnecessary because of the above changes;
- reduces lines taken by comments.

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!