Opt-in blinking tray icon on new messages

**Is your suggestion related to a problem? Please describe.**
People often miss new messages in Element, the notification popup disappears quickly and the red circle is small and doesn't attract their attention. As a result messages stay unread for a long time which makes Element unreliable.

**Describe the solution you'd like**
Some other chat clients like Gajim (XMPP) or HexChat (IRC) have animated tray icons that blink on new highlights and other important events so the user knows there's something that needs their attention. It's an optional feature as it distracts people easily but in corporate environments that's usually exactly what we want.

**Describe alternatives you've considered**
Add this blinking icon option, off by default so the client behavior isn't changed. Any time the red circle with a number appears on the icon it starts blinking as well if this option is turned on. When all messsages are read it stops blinking as the red circle disappears.

**Additional context**
Pull requests proposed:
vector-im/element-web#15015
vector-im/element-desktop#136
matrix-org/matrix-react-sdk#5135