Specific "Show/hide replace" tooltip for find/replace expander button

[JasonWeill]

Problem

Currently, the search and replace's expander button always has the tooltip, "Toggle Replace", whether or not Replace is visible:

People using screen readers would not know, without tabbing around to other UI controls, whether Replace is visible.

This is an issue as of Lab 4.1.0.

Proposed Solution

Have the tooltip be "Show Replace" when Replace is hidden, and "Hide Replace" when Replace is visible.

Additional context

This is adapted from #6650, sub-issue 2, bullet 4, filed by @tgeorgeux.