QSizeGrip: add optional mirrored resizing

I recently implemented a platform independent dialog inspired by the NSSheet dialog in OSX. The result came out fairly acceptable, but ... I was a little surprised how difficult that is with Qt (version <=5.5). I actually copied / reimplemented most of QSizeGrip's pimpl stuff to get it close to the resizing behaviour of NSSheet.

Although this might not be a "killer feature", some extensions to QSizeGrip would greatly help with such task.

What about adding some hints, that influence the resizing algorithm in QSizeGrip::mouseMoveEvent? Something similar to this enum:

class QSizeGrip {
public:
  enum ResizeHint {
    ResizeDefault = 0,
    ResizeMirrorHorizontal = 1,
    ResizeMirrorVertical = 2
  };
  // add QFlags definition for combination
};

Now when calculating the new size - which is done in mouseMoveEvent - the delta width/height can be mirrored to the oposite edge of the associated parent widget - still respecting the d->dxMax and d->dyMax values.

If you give me some introduction how to provide a request for Gerrit, I'll happily implement this and provide my first contribution to Qt.