Metamask extension icon should indicate which network is currently in use

**What problem are you trying to solve?**
When using Metamask, I would like to know what network I am on at a glance; without having to click the extension to open Metamask.

**Describe the solution you'd like**
I would like for the metamask extension's icon to change based on the network I am using.
The current icon should represent Ethereum mainnet, the other networks could be represented with the addition of a coloured dot to the metamask icon; matching the coloured dot in the network selector of the extension.

i.e. a pink dot would represent Ropsten, a Purple dot would represent Kovan, a Yellow dot would represent Rinkeby, a Blue dot would represent Goerli, and a grey dot would represent localhost/custom RPC.

Start by locating the browser extension icon handling and the network selector state used by MetaMask. Trace how the active network is exposed to the extension UI, then verify that Ethereum mainnet and the named networks show the requested icon or colored-dot states.

Written by the indexing model from the issue text.