Improve the strings of Tails Installer

Since we've forked liveusb-creator, we've customized it and improved our terminology so much that many user visible strings sounds outdated right now. As we'll bring more people to Tails Cloner in our updated backup plans, it would be good to review all these strings and improve them all at once (instead of bothering translators with too many incremental changes).

For example:

• Synchronizing data on disk...
• Lack of ellipsis
• Use of device
• Use of live image and other generic terms