Implement the XDG base directory specification properly

[zimward]

Problem

Currently the search paths are hard coded, which leads to failing of the mime db if the system doesn't place it at the canonical location (for example nixos).
Looking into the XDG Base Directory Specification it states:

There is a set of preference ordered base directories relative to which data files should be searched. This set of directories is defined by the environment variable $XDG_DATA_DIRS.

Implementation

The tests all pass with this implementation (and should do so on all systems, as long as XDG_DATA_DIRS is properly set up. [citation needed] :) ). As home crate is also not needed anymore (as these paths should also be in the env var) it is therefore removed from the dependencies (Fixes: #22 ).

[mbrubeck]

Thanks for the work! Just needs a few minor additions.

[mbrubeck]

Looks good!