Support usage of multiple accounts simultaneously across tabs

[markmhendrickson]

Currently, if a user opens up separate tabs for the extension and toggles to different accounts between them and starts taking actions (e.g. sending STX), they're likely to encounter issues since the wallet keeps state globally using only one account as once.

This enhancement would modify state so that it's tab-based so users can manage several accounts within the same browser at once.

This initial came up as a need by a user who wanted to send BNS names between their accounts efficiently. See on Discord

[kyranjamie]

This state should be ephemeral to each frame. The "currentAccount" state is in global storage atm. This should become lastUsedAccount or something.