Update visual.c to use Windows themes rather than the stock Raleigh theme,
and avoid hardcoding data paths for Windows (and Mac).  As the dlfcn.h
functions are only used when Python is enabled, move its inclusion there[*].

Also ensure that variables are declared on the top of the block.

[*] Python support Windows needs to be investigated, as POSIX signal
    handling is used there.

https://bugzilla.gnome.org/show_bug.cgi?id=730236