Use XDG_* variables to save config/state

[bbhtt]

This makes it fail to read/save settings under the sandbox

liferea-plugin-studio/webkit-setting/webkitsetting.py

Lines 157 to 162 in bebd17d

	config_dir = os.path.expandvars(
	"$HOME/.config/liferea/plugins/webkitsetting")
	cache_dir = os.path.expandvars(
	"$HOME/.cache/liferea/webkitsetting")
	data_dir = os.path.expandvars(
	"$HOME/.local/share/liferea/plugin-data/webkitsetting")

as Flatpak has no access to /home/user/.{config, local, cache} and the HOME variable is unchanged inside the sandbox, only XDG_* variables are set.

Instead use XDG_* variables which if not set falls back to $HOME/.{config, local/share, cache}

I haven't checked all the plugins but presumably anything that doesn't use XDG_* variables probably needs to be updated.