XDG base spec implementation

[dgcampea]

Summary of the changes in this pull request

Implements XDG Base spec.
Under Windows it tries to do something similar by using APPDATA env var.
Addresses #404

Pull request checklist

• I have added an entry in CHANGELOG.md including my name and issue and/or pull request number.
• If applicable: I have removed the corresponding entry in TODO.md.

[dgcampea]

ping @jendrikseipp

[jendrikseipp]

I appreciate you looking into this. However, I'm not sure if the change is worth it, since it makes user support more difficult. Currently, I can tell users where to look for their files. With the XDG spec, I won't know where the files are, even if I know which version of RedNotebook version they're using. I'll have to think a bit about this. What are your arguments for making the change? Could you please provide a table of before and after for all directories, the config file and the log file (using the XDG_* names) and a mapping of XDG_* variable names to their most common values?

[dgcampea]

What are your arguments for making the change?

XDG base lessens the clutter to the users directory and allows users to retarget the directories for storing their configs (instead of forcing a specific choice on the user).
Flatpak for instance manages the XDG variables too, to allow sandboxing/app config separation.

	Before	After	After (default XDG vars)	After (flatpak)	Fallback
Settings	~/.rednotebook	$XDG_CONFIG_HOME/rednotebook	~/.config/rednotebook	~/.var/app/app.rednotebook.RedNotebook/data/rednotebook	~/.config/rednotebook
Temp files	? ~/.rednotebook/tmp	$TMPDIR/rednotebook-<random string>	/tmp/rednotebook-<random string>	~/.var/app/app.rednotebook.RedNotebook/cache/rednotebook-<random string>	? depends on python tempfile
Log file	~/.rednotebook/rednotebook.log	$XDG_STATE_HOME/rednotebook/rednotebook.log	~/.local/state/rednotebook/rednotebook.log	(not yet implemented by flatpak) ~/.local/state/rednotebook/rednotebook.log	~/.local/state/rednotebook/rednotebook.log
Help file	~/.rednotebook/tmp/help.html	$XDG_CACHE_HOME/help.html	~/.cache/help.html	~/.var/app/app.rednotebook.RedNotebook/cache/help.html	~/.cache/help.html