Bug fix on Tools > Update mechanisms.

The menu now has a new state that reflects the moment where we check the update message.
In the past it wasn't possible to check and start an update if automatic update downloads were disabled.
The UpdateMediator.isUpdateDownloading() logic was a hack. Implemented it correctly by asking the InstallerUpdater if it was downloading an update or not. This meant adding a static state variable througout the download logic, both on bittorrent and http update download.
Added other fixes signaled by static code analysis warnings to avoid possible bugs.