feat: built-in files and editor Actions

Part of #61. Covers feature 3 (presets for well-known programs).

What

Two built-in Actions, bound by default and overridable or unbindable (key = "") by name:

Action	Key	Runs
`files`	`ctrl+o`	The file manager on the Project: `xdg-open` on Linux, `open` on macOS. Detached.
`editor`	`ctrl+e`	`$VISUAL`, else `$EDITOR`, else `vi`, on the Project path. Not detached (terminal editors need the tty).

Both are listed as built-ins and appear in the key hints.
The platform choice for files is tested without launching anything (inject the runner).
Overriding run or key and unbinding each one are tested.