Add support for `System Tray`

[sauntor]

Problem

Some apps employs System Tray for it's design purpose, i.e. Flameshot. If there's no tray, some features may be imperfect.
Take Flameshot for example, you can't start a snapshot from the panel via the tray.

The community plugin https://github.com/MvBonin/wingpanel-community-indicators can't work well, as for some apps does not shown in it's UI.

Proposal

By now, many utilities/apps use System Tray for their design purpose.

1. Add the official support for System Tray on the top panel.
2. Or add an extra area on the dock to simulate it.

Prior Art (Optional)

No response

[lenemter]

elementary OS will not support system tray, see elementary/wingpanel#96