Handle ToggleShowDesktop for widget-like windows (conky) properly

[Caellian]

Adding this small heuristic helps client_should_show properly handle widget-like apps (e.g. conky).

I don't think it causes issues with any other applications because in majority of cases skip_taskbar, skip_pager and below are really strong hints the app isn't showing as a normal window. The only thing that comes to mind that breaks this assumption is installer backgrounds that were common around 2010 (in which case hiding the installer too would be a bug), but those are not something we often see on Linux.

Ideally we'd have a widget WM_HINT, but that won't be added to X11 at this point lol.

Fixes issue reported in conky#2304.

[Caellian]

For reference:

Following should be used for openbox:

own_window_type = 'normal', -- or 'dock'
own_window_hints = 'undecorated,below,sticky,skip_taskbar,skip_pager',

One may use desktop window type if they use feh/notrogen, but for most other background apps, desktop will cause focus dependent window re-ordering.

With this patch, normal and dock are left as a simple binary decision regardless of other apps - depending on whether the user wants fullscreen apps to draw over conky or not (respectively).

desktop is really not the best window type choice. It has quirks with almost every wallpaper and desktop icons application besides the two mentioned above.