XDG User directories not set on fresh install.

[arshubham]

XDG Directories like XDG_CONFIG_HOME, XDG_CACHE_HOME, XDG_DATA_HOME are empty on a fresh install. This causes checks some scripts like nvm and yarn to fail and use home directory to dump config and cache files in home directory. See https://github.com/creationix/nvm/blob/8542df4ac503b6ee0dd055a67b07a88031e1804f/install.sh#L9

Currently granite itself sets the appropriate variables when needed, so I don't think there will be any problem setting them by default. https://github.com/elementary/granite/blob/master/lib/Services/Paths.vala