Follow the XDG standard for storing u2net models

[meequrox]

It is a bad practice in GNU/Linux to keep some dot files in the home directory.

There is an XDG standard that says that the directory to store user data files is defined by $XDG_DATA_HOME variable.

After this refinement, the order for selecting the models directory is as follows:

1. $XDG_DATA_HOME/.u2net (almost always ~/.local/share/.u2net)
2. ~/.u2net (if $XDG_DATA_HOME is not set)