macOS: Simplify and correct style mask determination

This function had accumulated a fair bit of accidental
complexity over the years.

- No early returns, make sure to preserve fullscreen
  state for all windows.

- Use windowIsPopupType() directly to set borderless
  for Qt::Popup (but not Qt::Tool).

- Handle Qt::Tool explicitly.

- Deduplicate Qt::CustomizeWindowMask handling.

- Remove case that used the absence of NSResizableWindowMask
  to remove the maximize button. Maximize is now
  disabled elsewhere (setWindowZoomButton). All windows
  now get NSResizableWindowMask by default.

- Qt::ForeignWindow now gets a standard window style
  mask instead of NSBorderlessWindowMask. The old
  code did not handle this case and left the mask
  value unmodified.

Change-Id: I56499e9f05c3f481b5a96e0507da2fb195f207fa
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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