Add apk backend support to PackageKit

[PureTryOut]

Off-topic: Maybe we should have a different repo for issues unrelated to pmbootstrap itself...

We should get apk support into PackageKit. PackageKit is used in combination with some other programs to offer a user-friendly front-end to package managers. An example of this is Plasma's "Discover": a software centre installed by default on the reference Plasma Mobile image which is creating a mobile layout. It would be awesome if we could use applications like these as the graphical way to install applications onto a postmarketOS system.

Some information to get somebody started.