Toggle Thunderbird when clicking on tray icon

[Vampire]

If the `mail.biff.show_tray_icon_always` option is enabled, it would be nice if the tray icon would support three cases:

- TB is minimized => show TB
- TB is not minimized but not focused => TB is brought to front
- TB is focused => TB is minimized

Currently, the first two options are already done like that, but in the last case clicking the tray icon just does nothing.

It would be awesome if in that case TB is minimized to the tray.
This way you can for example click on the tray icon, have a quick look on the situation, and right away hide TB again by just clicking again where your mouse already is, if nothing further is to be done right now.

[Jon]

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