A few small tweaks of the SecondaryToolbar code

This patch makes the following small changes to the SecondaryToolbar code:
* Passes in a reference to `PresentationMode`, instead of using the global.
* Removes the `isOpen` getter, since it should be sufficient to just check `SecondaryToolbar.opened`. This is also analogous to, for example: `PDFFindBar` and `PasswordPrompt`.
* Adds a check that the `buttonContainer` is defined when `setMaxHeight` is called. This prevents an error when reloading the viewer in "Responsive Design Mode" in Firefox.