[e-users] Search dirs in E16 menu generator script

Hi.

E16 comes with the e_gen_menu python script to generate an application menu
based on application desktop files. It uses a predefined set of directories
for searching.

I would like to suggest that it also (or instead) searches in the data
directories specified in the XDG Base Directory Specification[1], available
in the XDG_DATA_HOME and XDG_DATA_DIRS environment variables.

[1]
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html