Cocoa integration - implement Qt::WindowModal file dialogs

Previously, for Qt::WindowModal dialogs  we were creating modal sheet with beginSheetModalForWindow:
and later (from exec) calling -runModal, essentially making a window modal dialog into (now) application
modal sheet, which is not right- it's centered relative to desktop and the jump from window modal sheet's
position (centered relative to a window) to application modal was quite visible.
Now, instead of [panel runModal] (== [NSApp runModalForWindow:panel])
we call [NSApp runModalForWindow:theRequiredWindow].

Change-Id: I793dc72c7d1fe96497bb35754f4d0eac9a5e00e5
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>