Conform to xdg directory spec

[aanunez]

Versions

RedNotebook version (make sure you have the latest version): 2.5

Operating system and version: Ubuntu 18.04

Expected behavior

config and data should be stored in ~/.config/rednotebook and ~/.local/share/rednotebook

Actual behavior

Currently using ~/.rednotebook

Steps to reproduce the behavior

Use linux system

Log output when reproducing behavior

N/A

[jendrikseipp]

This is related to #159, which is concerned with the choice of directories under Windows.