Fix errors in handling of maximum field in wxGenericProgressDialog.

Fix errors in handling of maximum field in wxGenericProgressDialog.

Provide a SetMaximum() function for setting just m_maximum and return its
value from GetRange() instead of using m_gauge->GetRange() which doesn't work
when the native MSW version is used nor when the range is > USHRT_MAX under
MSW in any case.

More generally, this should fix a lot of bugs for progress dialogs using such
range as the values were not interpreted correctly in many places.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65506 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775