Correct lookup of Explorer-specific file association information.

Correct lookup of Explorer-specific file association information.

The code added in r52154 never worked because it was looking for the Progid
value in a wrong place, look for it under UserChoice subkey where it really
is.

Also add a way to look up the command to open files with the given extension
to the exec sample.

Closes #12302.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73106 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775