OperatingSystemView: handle missing default wallpaper

[danirabbit]

• Fixes a deprecation warning
• Use GLib to get system data dir
• Don't assume that a symbolic distributor logo exists
• Don't assume that the default wallpaper exists
• Fall back nicely

[danirabbit]

@bobby285271 With this, would it make sense to drop the wallpaper build option or do you still want it?

[bobby285271]

It looks like Gtk.IconTheme.get_default ().has_icon (logo_icon_name + "-symbolic") is false for us, so the build option should be a no-op for us right now (i.e. always no wallpaper). I cannot speak for the future and also cannot speak for other distros though.

Having elementary default wallpaper + NixOS default logo will be like: Not sure how other distro think about this, but yeah I personally do think our logo doesn't play nice with the circle right now - our default logo is not a square one ([502×435px](https://github.com/NixOS/nixos-artwork/blob/master/logo/nix-snowflake.svg)) and also not a white one.

[danirabbit]

@bobby285271 Yeah it shouldn't be possible to get that combination with this branch. The wallpaper background would only show if both a -symbolic version of the logo is available and a wallpaper matching elementaryos-default exists in system data dirs.

We can look at changing the name of that default in future branches if there was interest from downstreams in shipping a symbolic logo, but I'm thinking that this should do the thing you want without requiring a build flag. So if that makes sense to you, I'll remove the build flag in this branch as well

[bobby285271]

:ok_hand: Dropping build flag should be fine to me