- `percent_field` should be declared on `platypush.schemas.dataclasses`
level, since it's not specific to the `system` plugin.
- Added a common `SystemBaseSchema` that takes care of calling
`_asdict()` if the object is passed as a `psutil` object instead of a
dict.
Plus, `platypush.schemas.system` has now been split into multiple
submodules to avoid a single-file mega-module with all the system
schemas definitions.