Support all modality types in QPlatformDialogHelper

Support all modality types in QPlatformDialogHelper

Modality is not a boolean property in Qt. There are 2 types: window
modality and application modality. Native dialogs can support both of
these types as well (e.g. on Cocoa, a window modal file dialog should be
displayed as a Sheet). Remove the QPlatformDialogHelper::ShowFlags enum
and instead pass a Qt::WindowModality parameter to
QPlatformDialogHelper::show_sys().

The Windows implementation has been updated to check for
Qt::ApplicationModal instead of the ShowModal flag (since only
Qt::ApplicationModal dialogs are blocking).

The Cocoa implementation has been updated to only use non-modal and
application modal native color and font dialogs (which restores Qt 4
behavior). These are shared Cocoa panels that cannot be shown as sheets,
however. If the programmer asks for window modal color/font dialogs, we
use the Qt versions, not the native ones. The file dialog can be shown
either as a Sheet (but we need to pass an NSWindow parent for it to work
properly) or as an application modal dialog.

This change has been tested on Mac OS X with
tests/manual/windowmodality.

Change-Id: I9064987433895c55f68aac979ef8e8207fb24bbe
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>