Add preference for soft tabs

Add preference for soft tabs for #6872. Preference name is: "softTabs".

Only lines I added at the top of `Editor.js` was:

```
        SOFT_TABS         = "softTabs",
```

and:

```
    PreferencesManager.definePreference(SOFT_TABS,         "boolean", true);
```

The rest was re-ordering alphabetically. Sorry, I should have done that in a separate commit.

@TomMalbran You've added a few new prefs lately -- can you review this one?