Fix updater

[Gatsik]

• Recently server has started requiring a User-Agent in headers when accessing contents, so add this by using another function
• Also, this removes urllib usage from the client completely (it is still used for parsing links though), which is probably resolves #432
• Add some info to the VaultDownloadDialog as server has stopped to send Content-Lenght completely, so user will know at least amount of downloaded data and download speed