[rfc][icedtea-web] fix for RH947647, following the XDG basedir specification

[Jiri Vanek]

...
>>>
>>
>> Yes.  If users customize it, they will have to deal with the fallout.
>>
>>
>>> - how about xdg not following systems (windows) ? Keep .icedtea or move to
>>> ./config and ./cache ?
>>>
>>
>> Hidden directories is not how Windows programs stores its profiles.  You
>> might consider following what glib helper functions do
>>
>> Refer to
>> https://developer.gnome.org/glib/2.34/glib-Miscellaneous-Utility-Functions.html
>>
>> g_get_user_cache_dir() and so on
>>
>
> I didn't realise glib supported this for so long (since 2.6).  It seems like an
> excellent solution to just use that.
>

Hm... Im' wondering how to use this function cleanly from java...

I think that we should use the value of XDG variables or default ~/{.config .cache}/icedtea-web and 
_everywhere_. Even on systems like windows which do not support this.

As if we do this difference now, possible change of filesytem structure for some far future 
icedtea-web x.y.z , will be quite complicated.

J.