Make it easier to configure the distributor logo in the info-overview panel

Since this panel was redesigned for GNOME 3.36, the operating system name is no longer displayed directly beneath the distro logo. Because of this change, now downstreams want to switch to logo variants that include text, instead of just displaying an icon. (Icons don't usually include text.) And of course, when text is included, we need to accommodate dark mode as well. So I've added new meson build options to define filesystem paths from which to load the logos, one for light backgrounds and one for dark backgrounds. (Note GResource cannot be used here because some downstreams require logos to be packaged separately from gnome-control-center.)

I reject the approach of using /etc/os-release proposed in this bug because I'm not interested in adding stuff to /etc/os-release. We need specific control of the logo variant that we use in specific places. Attempting to define logos in /etc/os-release does not scale and is going to require adding too many logo variants there (one logo for gdm, one logo for gnome-control-center, one logo web servers, etc.)

This merge request does contain one improvement for the /etc/os-release codepath though: it changes the size of the icon from 256x256, which is waaaay too big, down to 128x128, a more reasonable size. This should benefit distros that don't use these new build options to define specific logos.

Finally, note that I intend this MR for GNOME 41, since GNOME 40 is frozen to UI changes, but it would benefit from preliminary review now since this patch is going downstream immediately.

Before (logo from /etc/os-release)	After (logo from /etc/os-release)	Example with custom logo defined	Upstream branding is unchanged (before/after)

CC @aday