If the user has enabled the XDG user directories module then we can
use the XDG music directory in the MPD module. Otherwise we'll leave
the option undefined so that the user is forced to define the
directory to use.
This applies to state version 22.11 and above.
Fixes#3225
This brings a few advantages:
- Use of float instead of strings to represent float values,
- Use of structure settings, and
- Better type checking for some settings
Also add thiagokokada as codeowner of picom.
Remove `stateVersion`, `username`, and `homeDirectory` as they can be
set in the configuration directly. Together with the previous commit,
this makes setting `stateVersion` explicitly mandatory.
Also replace `configuration` by `modules`, which expects a list of
Home Manager modules.
Also remove `system` which was made useless by #3028.
The user should always explicitly set the state version they wish to
use. Indeed, the configuration generated by the Home Manager install
script has set this option for a long time. This removal should
therefore not affect many users.
