Fix layout of main toolbar

Before this commit there were two main issues:
- In small windows, the zoom controls visually floated above the page number
- In small windows, the (transparent) zoom container covered the go-to-page
  input box, which prevented one from using the input field to quickly
  navigate to a different page.

Also reduced the specifity of the CSS selectors targetting `.outerCenter`, because I did not find any reason for using specifity 0,1,3,1 over 0,0,2,1. It only added unnecessary complexity of maintaining the style sheet.