Clipboard countdown bug(s)

[lcmaqueda]

• The clipboard-clear countdown is started when selecting an entry, even if OS dialog for unlocking the key is canceled.
• The clipboard countdown is not restarted if a new entry is selected.

[lcmaqueda]

There are also a couple of potential risks in the existing logic:

• A potential race condition when checking if the counter was running
• There is no limit in the number of gorutines that could be spawned to initialize the counter: holding the enter key over a password entry begins spawning goroutines without control that could result in a crash of the application or even the OS.