Further Download related cleanups

* Moved the memory buffer and mutex handling into the Download class to
  simplify the code updating the UI in ServerDialog and UpdaterWindow.

* Replaced the "DownloadUpdate" callback function with simply polling
  Download::getState, since in the end polling was happening anyway.

This changes also fixes handling of the Enter key while downloading
updates, which no longer cancels the update process. Also, when pressing
Escape while things are being downloaded, the first press cancels and
only the second press goes back to login.

Introduced a ThreadSafe template class, which wraps any type and makes
it only accessible by calling lock(). This ensures the data is never
accessed without locking the relevant mutex.

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