[desktop] Notification badge

Summary: Electron provides easy way to set the badge. This diff sets it to the unread count whenever it changes.

Test Plan:

• Send a message from another account and check if the badge is displayed
• Read the message and check if the badge disappears

Reviewers: tomek, atul, abosh, kamil

Reviewed By: tomek, atul

Subscribers: kamil, ashoat

Differential Revision: https://phab.comm.dev/D5633