Ensure that names of saved XPMs are valid C identifiers.

Ensure that names of saved XPMs are valid C identifiers.

The name of the XPM should be based on the file name but can't always be
exactly equal to it as it's possible (and relatively common, e.g.
"foo-24.xpm") to have characters forbidden in identifiers to appear in the
file names. Ensure that we use a valid C identifier here.

Closes #13905.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72029 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775