Change settings - rename and move to global settings

Up to now, all package settings were set in Dart.sublime-settings
files. This was undesirable, as they would only be visible to
*.dart files. Thus, for example if you started ST and pubspec.yaml
was the active file, some core settings of the package wouldn't be
visible. Dart.sublime-settings must only contain settings
affecting the .dart file type directly. All other settings must
be made global.

- Rename dartlint_... settings to dart_linter_...
- SDK now finds the sdk path via Preferences.sublime-settings
Fix error hierarchy
Remove command to open default .dart settings

We don't include those in the package any more, so it doesn't
make sense to have it.

- Remove command to open default .dart settings
- Refactor
Revert "Remove command to open default .dart settings"

We actually need a variant of this.

This reverts commit e1f0d06b658ea04400326f3dcfd445b1f9d2b337.
Reinstate command to check default settings

- Refactor
- Reinstate command to open default settings (global)
- Small changes to make it obvious to users that they should not
  edit certain settings files.