QFileDialog: no way to set icon provider before creating widget

In some rare cases (e.g. some versions of Dell Backup software), system-provided file/folder icon provider does not work with 64-bit Qt apps (something wrong within Dell's DLL resulting in crash, may be requested icons size are not supported, do not know).

To avoid this problem (large number of users are affected, because Dell Backup is commonly installed on many Dell laptops), we use own icon provider to represent file/folder icons.

Unfortunately, this does not works from scratch with QFileDialog: icons are queried on createWidgets call (called from setOptions()), resulting in Dell DLL crash. There is no way to avoid this problem, because setIconProvider works only after widget are created.

Attached patch fixes this problem, it allows setIconProvider call before setOptions(), if icon provider is set, than system (buggy) icon provider not called.