Treat application settings differently

[FichteFoll]

There are some settings that can only be set in Preferences.sublime-settings, such as theme or overlay_scroll_bars and similar, that affect the entire application across all windows. These settings should not be considered for completions in syntax-specific and other settings that boild down to a per-view level.

In fact, we can even mark these settings as "unknown" in these other settings files, although I would try to annotate that the settings aren't misspelled but misplaced.

Will probably need to be a hardcoded list of settings.