WIP: Implementation for pmb.api

This is a WIP implementation for a pmb.api module that makes it saner for external software to interface with pmbootstrap. This is needed to create a tightly integrated GUI frontend on top of pmbootstrap.

Things the pmb.api module does:

• abstract away the args variable thats passed around inside pmbootstrap and work more object oriented instead.
• return predefined datastructures instead of dicts (which is neater to work with in external code and can contain inline documentation)

This code is split from !1701 (closed) (and there are a few comments on the code there that needs to be fixed here)

Example usage:

import pmb.api

for deviceinfo in pmb.api.list_deviceinfos():
    print(deviceinfo.name)