In issue discussions, tests and various parts of the code there are
mentions of side-by-side mode being active or not. The `Guest` class
didn't store this state directly but instead had a
`closeSidebarOnDocumentClick` state which is always the inverse of
"is side-by-side active?".

Make the code easier to follow by storing the "is side-by-side active?" state
directly. This will also avoid a misnamed variable if in future the
state has other effects besides changing whether clicking on the
document closes the sidebar.

 - Replace `closeSidebarOnDocumentClick` with a private
   `_sideBySideActive` property, exposed via a read-only
   `sideBySideActive` getter

 - Refactor tests to only use the public API of Guest (calling
   `Guest#fitSideBySide` or reading `sideBySideActive`)