applet: add plasmoid status (conditions for shown when relevant)

We have a lot of tray icons and in order not to overwhelm users and keep focus on the important bits, we should only show items that require their attention and/or invite direct action.

We recently did something similar with the battery applet to show it as passive when the laptop is plugged in and no special power mode is active.

Show the applet's status as active if

• there are connections available but none are active (to offer easily accessible actions), or
• airplane mode is enabled (to remind the user of special state).

Conversely, this means that the applet gets hidden by default when

• there is an active connection (this is the "normal" state and there is not much for the user to do in this case)
• there are no available connections (there is not much for the user to do).

Note that users can still opt to set the applet to always shown if they wish to, and even when not shown in the panel the applet is still only one click away in the dropdown button to see more details or perform actions.

@teams/usability