Add logging option

[Narrat]

Is your feature request related to a problem? Please describe.

There may be the case, that the gpg setup or keyring causes problems within gpg-tui. Without any visible output such an issue can be difficult to pinpoint.
See for example #135

Describe the solution you'd like

An option or cli argument which enables some sort of logging. With this enabled gpg-tui provides information what it is doing behing the scenes and may help to pinpoint issues.

Possible implementation can look like this

Describe alternatives you've considered

There is no real alternative? STDERR output directly into the terminal isn't viable with a ncurses TUI.

[orhun]

Implemented in #201 - now you can press CTRL-L / F2 to see the application logs. Let me know if we need more verbosity!