Fix #143 - Rename Dart.sublime-settings to Preferences.sublime-settings

With this change, users will be able to control most settings of
the Dart package from their User/Preferences.sublime-settings
file. That's probably what most users would expect.

Note: Sublime Text gives priority to syntax-specific settings
over plain settings, so the previous Dart.sublime-settings file
would shadow any changes to User/Preferences.sublime-settings.