[CMake] is there an equivalent property to source_group command?

[J Decker]

After some searching, I finally found the source_group command, (which
requires groups to be specified with '\\' unless the delimiter is
redefined somehow;  The output to the project file is a '/' *grin*
Isn't there a generic is_path_character() function available? ).

How hard would this be to implement as a property on source files
instead?  Maybe it is already and I missed it?  Would think that
FOLDER as a property on source files would be excellent.

The other problem is that this doesn't really compare the files the
same way that source targets are... so if I specify the file as
'src/file' and the file is actually 'file.c' then the file doesn't get
put into the source group...