Allow Beam wallet to manage multiple wallets

I didn’t see any formal FR for it, but I understand it’s a common request.

Even if Beam transactions are confidential, and even if different addresses can be used each time, it would be useful if the Beam wallet app itself could manage multiple wallets (this request is mainly for the Desktop GUI wallet).

The switch between wallets would be done at the very start of launching the app. And that initial window would also have all the needed options to manage these wallets (create new wallet, delete, rename, etc.).

I guess this feature would imply handling multiple wallet.db files, etc. Thus, each wallet would need to have at least a name defined by the user. And optionally a description, if possible.

I don’t know if the information about the name could be stored within the wallet.db file itself (although, if so, it should be visible without needing the password). Maybe other methods would be needed (like using different filenames: wallet-[some hash or random number].db and then have a list of all these files, with their associated names and descriptions, in the beam-wallet.cfg).