[desktop/mac] make destroying ipc more robust with destroyed webcontents

[ganthern]

sometimes, the window is gone before we get to destroy the bridge. This gives a TypeError: object has been destroyed. In other cases, we try to send messages to already destroyed webContents, giving TypeErrors because webContents.fromId returns undefined.

To prevent this

• we add a listener to the destroy event of the webContents
• don't use ids for referencing the webContents, instead pass the content itself around to be able to check isDestroyed()

fix https://github.com/tutao/tutanota/issues/4467