Added priority to also count inside files

This is a extension of the file order patch. It extends the priority system to within files with the following rules:

The first element of a file has the highest priority, the last element has the lowest one (namely 0). Also mappings can get a `{priority, X}` setting passed that overwrites this automatic rule to force a mapping to have a specific position inside a file, this comes in handy to decouple position in the generated `.conf` file from the priority of a setting withn a file.

In file priorities take a lower impact then the permissions assigned by the file order, this prevent schema orders to mess with other schemas. (i.e. everything in `00.schema` will always have a higher priority then `01.schema`).

# WARNING

This breaks some of the current behavior as it seems that as of this moment the last element in `.schema` to have the highest priority. I left the test failing to point this out.
Other then that I think the new behavior is more sensible since in other places the 'first has higher priority' is already working this way (i.e. the 00.schema lads first in the .config etc.)