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/branches/WX_3_0_BRANCH@75605 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Document incompatible change to wxWindow::FindWindow() in 3.0.

We now skip the TLW children in this method, see r74721.

Also explicitly mention that TLW "children" are not searched by this function
in the documentation.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75607 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775