Settings: Make shell selection fields show full path

[Taitava]

• For built-in shells, it's clearer if the selection shows e.g. /bin/bash instead of just Bash.
• For custom shells, no changes are needed, it's good to show just the name defined by a user.

This change could make it clearer to see which shell gets invoked in situations like #358 - although it doesn't actually fix a problem why a wrong shell path is used.

Also need to ensure that OS's default shell selection shows full path, not just the shell's name.