Fix native dialog save/open on macos 10.15

If native FileDialog is set as "modal" the dialog exec
method will always return "0".

Is we are in macOS and Native FileDialog is set we
avoid setting the dialog as Modal as native FileDialog
in macOS always behave as a Modal Dialog

BUG:413241
(cherry picked from commit e27ab3a7382dcf3c6f877d6c949dc13c550539f7)