bug#51404: Support system dark mode on Windows 10

[Vince Salvino]

So, playing around with the idea of dynamically toggling dark/light modes 
on-the-fly, rather than only at program initialization as this patch currently 
does. I got a proof of concept working, however there is one ugly caveat. In 
order to change the theme, we must keep track of every window handle in 
existence (frame, scroll bar, other UI element) in a global.

The code for this is relatively simple, we can watch for WM_SETTINGCHANGE (in 
w32_wnd_proc) and then call w32_applytheme (which also needs a few 
modifications) to all of the HWND objects. But it seems like it would get quite 
messy and bloated to be storing references to all of these in globals. We would 
probably need some kind of array of references, and then loop through them upon 
receiving the signal.

The pseudo-code is something like:

        // Add every hwnd in existence to this array.
        global_hwnds = []

        w32_wnd_proc() {
                ...
                case WM_SETTINGCHANGE:
                        for each hwnd in global_hwnds {
                                w32_applytheme(hwnd);
                        }
                ...
        }

Thoughts? Is it worth it?

Vince Salvino