Quote file names with spaces in wxFileType::ExpandCommand().

Add double quotes around the file name inserted into the command to open the
file by wxFileType::ExpandCommand() if the file name contains any spaces and
if it's not already quoted by the command line itself.

While this doesn't completely fix the problem, it does help with opening the
files with spaces in their names under Windows and shouldn't do any harm under
Unix.

Closes #4607.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64655 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775