API/providers refactorisation
-put all access to thetvdb and trakt in 2 classes (one of thetvdb and one for trakt)
-create and improve diskcache class (cache for xml and json files from the two providers)
-integrate these api classes into series, serieslist
-change thetvdb parsing and seen parsing/creation files to do all xml parsing and creation with recommanded QXmlStreamReader and QXmlStreamWrite from Qt core instead the Dom from Qt xml module
-remove libQt5Xml.so.5 file from deployment
-remove now useless actorlist class