Previous taglist_get_tagstring signature/implementation did not allow
handling of cases where inputted buffer could not contain all tags.
New implementation allocates buffer based on pre-computed size allowing to
insert all tags in the returned string.
Added get_taglist_string in qthelper to handle conversion to QString
Added TestTagList with tests for taglist_get_tagstring
Signed-off-by: Jeremie Guichard <djebrest@gmail.com>