Enhancement proposal: change icon when new mail appears.

[wl2776]

Ubuntu 18.04
KDE Plasma 5
Thunderbird 60.8.0 (64bit)

Currently tray icon remains the same all time.
It would be good to have an indication that new unread messages have been received, with changed icon.

[Temp345]

Please, this is a must-have feature. Linux Mint, XFCE, thunderbird 60.9.0 (64bit).