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.
Merge pull request #144 from my-personal-forks/master

v0.2500.0 - release preparation
Improve I/O scheduling for linter

The linter, while extremely useful, offers a rough experience and slows ST
down in some scenarios -- especially if you switch to a project with many
open Dart files.

This change addresses the perf problem and some user experience issues,
mainly:

Performance
-----------

Now there is a *producer* thread and a *consumer* thread exchanging error
data. They communicate via a synchronized queue.

The *error producer* fires when a view is activated or saved. I've
disabled linting *on_loaded* because it was causing serious perf
degradation.

The *error consumer* goes about its business in intervals. It checks the
queue for new items and acts upon them. Since it's an async communication,
a token and a buffer id is passed along with error data to identify the
most current lint operation for the buffer underlying the current view.
Results with expired tokens are discarded.

Experience
----------

No output panel is shown if there are no errors.
Merge pull request #148 from my-personal-forks/linter-2

Improve I/O scheduling for linter
Add command to open User/Dart.sublime-settings
Merge pull request #146 from my-personal-forks/settings

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