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

**TODO**

- [ ] Update docs
- [x] Revise commands to open settings - we may not need them now