Select profile when starting TUI

We should have an option to start the TUI via a specific command-line option. Further parameters can then be specified for TUI-specific actions.

One such further parameter is the option to specify which profile to load. If the user specifies a profile, the TUI will try to load it. If it fails, it will report an error and next behave as if the user had not specified a profile (or maybe should just shut down).

When a user doesn't specify a profile, the TUI will open with a list of the existing profiles, and also offer an option to create a new profile. If the user selects an existing profile, they will be prompted to enter the profile's password so that the profile can be decrypted.

If the user chooses to create a new profile, they will be taken to the "onboarding page" (which we should rename to something like "create wallet").

As an aside, profile basically equals "wallet" from my point of view and we should probably decide if we should continue to use terminology profile or just switch to the term "wallet" when talking to users.

#280 is a pre-requisite to this issue.