Simplify naming for settings files:

We'll now have this in the ST UI:

a) Preferences: Dart - Default
b) Preferences: Dart - User
c) Preferences: Dart - File Type (User)

(b) and (c) are user-editable.
(a) is read-only and not meant to be edited by users; only
    consulted for documentation, and used by this plugin to
    provide default values for settings.
Transplant User settings to new file.

This commit will take care of transplanting old settings to the
new settings file (not yet added).

A prominent message will be displayed to users, and they should
not need to take any further action on their part. The Dart
package for Sublime Text should continue to work transparently
after a restart of ST.
Add new settings file 'Dart - Plugin Settings.sublime-settings'

From now on, we'll stop using 'Preferences.sublime-settings' as
our main storage file for Dart settings, and will start using
'Dart - Plugin Settings.sublime-settings' instead.

This new file is now accessible from the ST UI:

- Using the command palette
- Using the main menu (Preferences > Package Settings > Dart)
Clean up code
Merge pull request #274 from my-personal-forks/fix-267-menu-item-for-user-settings

Streamline settings files