Add focus keybindings to the config

[Builditluc]

This PR adds the option to change the keybindings for the focus selection. It introduces two new configuration values. These are keybindings.focus_next and keybindings.focus_prev. The defaults and a sample usage configuration are below:

[keybindings]
focus_next.key = "n"
focus_prev.key = "p"

An example of how to use these new settings

[keybindings]
focus_next.key = "tab"
focus_prev.key = "tab"
focus_prev.mode = "shift"

The default configuration