Make it possible to update downloadable voices from new release

[lumpidu]

• if the old voice is deleted, the meta data is wiped from db and disk
• a new release info fetch from github is initiated
• the hard coded value DownloadVoiceManager::sReleaseName is used for fetching the voice release meta data
• the voice manager shows the updated entry from the voice db and the user can download the release just normally as before