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
  (e.g. 733px).
- 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.
Merge pull request #3671 from Rob--W/toolbar-layout-fix

Fix layout of main toolbar