add GTK3 cups support

[tinywrkb]

This tries to add the GTK3 cups backend taking advantage of the work that been already done for Chrome app.
I haven't tried building this, I'm letting the CI do it that, so this is still untested.
Closes #1.

[flathubbot]

Started test build 37250

[flathubbot]

Build 37250 failed

[flathubbot]

Started test build 37251

[flathubbot]

Build 37251 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/35856/net.cozic.joplin_desktop.flatpakref

[tinywrkb]

Argh... still broken, I will try to see if I can solve it this later.

[flathubbot]

Started test build 37382

[flathubbot]

Build 37382 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/35985/net.cozic.joplin_desktop.flatpakref

[tinywrkb]

@catsout This is working as expected with the limitation that the GTK3 print dialog will only appear if there's a running CUPS daemon.
It seems safe to merge but I'll leave it to you.
That's it for me today.