Support file associations using "show" verb under MSW too.

If we failed to find the "open" command, try the "show" one as some programs
(notable Microsoft viewers for Office file formats) only define the latter but
not the former.

Closes #15821.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75606 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775