Provide indication of locked / unlocked status on extension toolbar image

**What problem are you trying to solve?**
Currently the MetaMask logo in the extensions toolbar does not provide any feedback except for pending txs to sign. As dapp usage becomes more common, many of them require the user's wallet is unlocked and logged into.

**Describe the solution you'd like**
A simple solution is to provide user's visual feedback indicating the status of their wallet. A simple Grey locked icon on the bottom right corner of the MetaMask logo would suffice. And then when the wallet is unlocked, maybe a green unlocked icon.

**Additional context**
This could provide a security risk, if the computer is left open, someone could see the wallet is unlocked and click on it to access funds. Ideally this feature request would also be implemented with an additional "re-enter password on signing txs" setting to help prevent theft.

The issue names no files, tests, or entry points. Start by locating the extension toolbar image and the wallet lock state, then determine how the requested locked and unlocked indicators should behave. Done means showing the wallet status without exposing an unsafe security signal, with the proposed re-enter-password setting resolved or explicitly scoped.

Written by the indexing model from the issue text.