Focus terminal on enter, strip trailing spaces on doc focus out

[jeremypw]

Fixes #581

It is considered potentially problematic to strip spaces on autosave so this solution is proposed. Stripping only occurs when the document is unfocused on the assumption that in that case the user is no in mid edit.

It is inconvenient to have to both move and click the pointer to focus the terminal and the same to refocus the document. Also lint errors are often caused by failing to force save the document first (trailing spaces are not stripped during autosave).

• Implement focus follow between the terminal and the current document (does this need a setting?)
• Strip trailing spaces from the document when focused out (subject to the setting). Leave whether this change is saved to the AutoSave setting,

[ryonakano]

I'm not sure if we should reuse strip-trailing-on-save setting because this is not triggered on save.

[jeremypw]

@ryonakano Maybe we can rename the setting to strip-trailing-on-save-or-focus-out or just strip-trailing-spaces? This setting is intended to control whether trailing spaces are ever stripped; the fact that up to now this only occurred on saving (non-auto) is kind of incidental. I cannot see the advantage of having two separate settings tbh.