Ci, remove failure and add gtk_macros

See merge request !5

- Replace failure with anyhow
- Use gtk_macros instead of copied macros

- Uses 3.36 instead of master
- Stop using deprecated flatpak-ci-initiative-sdk-extensions.yml
- Whitespace changes

layout: use centered layout for 3.36

See merge request !4

- based on design team dicussions, we'd like the style updated to
  centered layout. This will be a multi-step process touching both
  Banner viewerer and Software (along with many banner assets).
- use typography styles laid out in GNOME/gtk#1808

Teams/Design/software-mockups#2

Preapre for a 0.0.1 release

See merge request !3

Gitlab CI: use sdk-template

See merge request !2

icons: proper app icon

Closes #1

See merge request !1

Fixes #1

it also cleans up the stylecontext errors before displaying them

We could use AppstreamGlib here to get the app info using their id
but that's too much work for now. appstream-glib-rs is now a thing, might revisit soon"

Don't build what we don't need

Currently, some banners might use external resources for legal reasons
downloading them sync one after an other is not great.
This adds downloading of external resources & caching them.

GTK CSS loads external resources that are supported by gvfs;
It's sync, so it's kinda slow & blocks everything