Always keep "Show SSH Manager" text, let checkbox represent state

Having a checkable menu item combined with changing texts is confusing. "Hide SSH Manager" in checked state would indicate that the hidden state is active, which is not actually the case.

The menu item text should either indicate an action, in which case renaming is okay but no checkbox should be added, or the unchanging text should name the state that's either active/checked or inactive/unchecked.

In the spirit of "Settings" -> "Show Menubar" / "Show Statusbar", I'm picking the latter, so the text always remains "Show SSH Manager" and only the checkbox state gets toggled.

CC @ngraham on whose blog entry I saw this and who encouraged me to submit this MR. Thanks to Nate for the visibility and @tcanabrava (assigned as reviewer, is that correct?) for the new feature!