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
The entire UI is constructed with templates, so the wrapper
constructors are never called, except for gtk_inspector_window_new,
which gets called from the GTK+ code.
Moving the inspector into libgtk lets use reuse internals without
having to add public API for everything or inventing awkward private
call conventions.
https://bugzilla.gnome.org/show_bug.cgi?id=730095
