This signal was not being emitted on macOS, a bug introduced in 941f3c38.
`emit_monitors_changed` was never set to TRUE, so the signal was never
emitted. `emit_monitors_changed` has now been removed, because its
functionality was moved to GdkDisplay.
In addition, kCGDisplayDesktopShapeChangedFlag was removed from the list of
flags that indicate a new/reconfigured monitor, because monitor removals also
include this flag, which caused removed monitors to not be removed from
GdkDisplay.
Fixes#2004.