Follow XDG dirs spec

[polirritmico]

Fix #643. Uses QStandardPaths to get cache, data and config XDG folders. If old folder (e.g. ~/.hydrogen/) is found, use that instead of the XDG paths.

According to https://doc.qt.io/qt-5/qstandardpaths.html this should be the paths returned by QStandardPaths by system (For Linux follow XDG env paths):

QStandardPaths	macOs	Windows	Linux
AppConfigLocation	~/Library/Application Support/hydrogen	C:/Users/USER/AppData/Roaming/hydrogen	~/.config/hydrogen
AppLocalDataLocation	~/Library/Application Support/hydrogen	C:/Users/USER/AppData/Local/hydrogen	~/.local/share/hydrogen
CacheLocation	~/Library/Caches/hydrogen	C:/Users/USER/AppData/Local/hydrogen/cache	~/.cache/hydrogen

The __usr_*_paths are setted outside a function, so QStandardPaths isn't avaliable to get the XDG paths. Also no APPNAME is avaliable until setApplicationName() is called after the Filesystem::check_usr_paths() is used. To avoid this update_usr_paths() is added at the begining of check_usr_paths() to update the correct values from QStandardPaths.