macOS: Fix GdkScreen::monitors-changed
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 (closed).