Implement text input control for settings

[BenBE]

For allowing the proposed feature in #637 to be user-configurable there needs to be some UI for the user to input some text, edit the existing value and possibly have the UI provide some validity feedback.

The new text input should extend the list of normal settings controls we already have for checkbox items and numbers by a new text input method. That text input method should allow for some basic validation of the user input as well as provide a sensible subset of the (basic) readline features for text cursor control, word/line editing and possibly tab completion (e.g. when the input setting is a list of paths).

The second part consists of an example implementation of this new control to allow for #637 to be user-configurable list of paths.

[cristianUrbina]

I am working on this issue

[JonasPfi]

I've implemented this in #1961

This adds a new StringItem settings control backed by LineEditor
and replaces the hardcoded path list in Process.c with a
user-configurable colon-separated string that persists in htoprc.
When left empty, the original built-in defaults are used as fallback.