Explicitly define layout of toolbar buttons

Before:
- Firefox's buttons looks OK
- Chrome (quirks mode): Buttons were aligned to the bottom (too much)
- Chrome (standards mode): Buttons were aligned to the top (too much)
- Opera/IE/Safari: Like Chrome (standards): Buttons too high

(Too high = Compare the other buttons to the rightmost button)

After:
- Firefox's button positions didn't change at all
- All buttons are aligned at the same level, across all browsers

Screenshots for comparisons:
https://rob.lekensteyn.nl/pdfjs/
https://rob.lekensteyn.nl/pdfjs/buttons-firefox-before.png
https://rob.lekensteyn.nl/pdfjs/buttons-firefox-after.png
https://rob.lekensteyn.nl/pdfjs/buttons-chrome-before.png
https://rob.lekensteyn.nl/pdfjs/buttons-chrome-after.png
https://rob.lekensteyn.nl/pdfjs/buttons-opera-before.png
https://rob.lekensteyn.nl/pdfjs/buttons-opera-after.png
https://rob.lekensteyn.nl/pdfjs/buttons-safari-before.png
https://rob.lekensteyn.nl/pdfjs/buttons-safari-after.png
https://rob.lekensteyn.nl/pdfjs/buttons-ie-before.png
https://rob.lekensteyn.nl/pdfjs/buttons-ie-after.png