Add support for IEC and SI size units to wxFileName::GetHumanReadableSize().

Add support for IEC and SI size units to wxFileName::GetHumanReadableSize().

Allow choosing the multiplier to use (1000 or 1024) and the names of the units
(SI or IEC). By default, still use traditional convention, i.e. multiplier of
1024 but SI unit names. This will probably need to be changed to use IEC units
in the future.

Also added unit test for this function.

Closes #10673.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63870 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775