Make applet active in the System Tray only when any vaults are open
Right now the applet is active when any vaults exist, and inactive otherwise. This lets you always access a vault with two clicks.
As a downside, the Vaults icon is always taking up space in the visible part of your System Tray. My sense is that most users of this feature aren't opening and closing vaults all day every day, so for these people, showing the icon 100% of the time wastes space.
Accordingly, let's make the applet active only when any vaults are actually open. This makes the visible contents of the System Tray more contextually relevant, and also provides a visual reminder when you've forgotten to close your last vault--the icon is still visible!
The drawback to this change is that accessing a vault when none are already open becomes a three-click task. I think this is an acceptable trade-off for the above benefits, and people who prefer the status quo can restore it by setting the applet to be always visible in their System Tray settings.
Implementation-wise, the change relies on storing a list of open vaults in the model that's kept in sync with the one in the KDED service, which mirrors what's already done for the list of busy vaults and vaults in an error state.