Use Adwaita as a fallback icon theme

[daudix]

Currently, if no fitting icon is found in elementary icon theme, gtk falls back to hicolor, as per the fd.o spec. With this PR, it will first try to find missing icon in Adwaita (which is present in elementary OS) before resorting to displaying a missing icon image. This issue mainly affects GNOME Circle apps that heavily rely on Adwaita, and while it is recommended to bundle all non-fd.o icons, not all apps do so (unfortunately).

Before/after examples (Apostrophe and Ptyxis):

[danirabbit]

I don't have any objections to this. Just need to double check what the behavior is here with gresourced icons from Granite and make sure GNOME icons don't override that

[daudix]

Went through all core and some AppCenter apps, as well as granite-demo, and elementary icons seem to always take priority over Adwaita.

[danirabbit]

As far as I can tell this works as expected and doesn't override icons gresourced in granite

[tintou]

It doesn't seem to be a good thing to me, if the icons are missing, it means that the app has to gressource the fallback icon

[danirabbit]

@tintou I agree they definitely should, but OP is right that a lot of apps don't do it :/ So it helps to be a little less broken for those apps at least. As far as I can tell it doesn't break anything

[tintou]

Every time that I report it, the app get fixed in a few days, I'm not sure that we are doing anything good with the ecosystem to apply this workaround just for us

[danirabbit]

@tintou hm yeah maybe you're right. If you want to add a PR to revert with that reasoning, I guess I'll revert. And we should probably add a comment to the theme file about why we don't have inherits.