Add AppIndicator3 and AyatanaAppIndicator3

[HeavenVolkoff]

Hello,

This PR adds GIR and VAPI definitions for libappindicator3. Used to create tray icons with gtk3.

https://launchpad.net/libappindicator

[ricotz]

This library got superseded by https://github.com/AyatanaIndicators/libayatana-appindicator

[HeavenVolkoff]

Hello @ricotz,

Thanks for reviewing this PR.

I added definitions for AyatanaIdo3 and AyatanaAppIndicator3. Should I also remove the definitions for AppIndicator3? I know it was superseded, however it seems to still be widely used and not completely abandoned (last commit was from 2020). Also, AyatanaAppIndicator3 is not yet available on all majors distros (e.g.: Fedora), and I have seen projects which do fallback builds with AppIndicator3 when AyatanaAppIndicator3 isn't found on the system, so maybe it is worth keeping it with a deprecated notice of some sort?