Symbolic icons only have the `-symbolic` suffix, breaking compatibility with FDO-compatible apps

[Here I use the copy icon as an example, but the issue affects everything]

The Adwaita icon theme has an icon-named edit-copy-symbolic for its copy icon.

The FDO icon theme name spec specifies that this icon should be named edit-copy; see https://specifications.freedesktop.org/icon-naming-spec/latest/ar01s04.html.

As a result there are 18 years of FOSS apps that ask the active icon theme for edit-copy, not edit-copy-symbolic.

When Adwaita is the system's default icon theme, and such an app asks for edit-copy, it's not found. Per the icon loading spec, the icon loader will chop off the last part and look for edit. That won't be present in Adwaita either. Hence no icon will be found, and the the app will have a blank space instead of a copy icon. Users will file bug reports, complain on Reddit, and make 20-minute YouTube videos crooning "Whyyyyyyyyyy??????" into a camera.

Now, I understand why Adwaita wants its symbolic icons to have -symbolic appended. In KDE we like this convention you came up with, and we're moving towards it as well, and porting our apps to ask for edit-copy-symbolic. However we retain the old icon names for backwards compatibility with FDO-compatible apps that have not yet adopted this convention.

It seems that Adwaita does not have these compatibility symlinks, or else removed them at some point. This is problematic because it breaks icon loading in FDO-compatible apps that haven't been ported to ask for -symbolic icons, which is to say nearly all of them.

Consider adding or re-adding compatibility symlinks, so that apps outside of GNOME and KDE aren't missing almost all of their symbolic icons when using Adwaita.