Currently, the Vaults icon is always visible in the System Tray. However most tray icons
dynamically show and hide themselves based on whether or not they're relevant and in use,
which results in a nice clean clutter-free System Tray. So it would be nice for Vaults
to do the same.
This patch implements that behavior by adding a `count` property that gets updated based
on vault creation and deletion, and making the icon only appear in the tray when the count
is greater than 0. People who prefer to have it always visible can of course override
this in the System Tray settings, but the auto-hide behavior in this patch is probably a
better default for most.