WIP: Add ability to navigate symbols

I'm adding a basic .tmPreferences file to help ST index symbols in
Dart source code. The indexing won't work right now because the
symbol patterns are limited to scopes that do not yet exist in the
syntax file. These should be defined.

Navigating Dart source code would better be implemented as in the
Dart editor by using the same source code analyzer, but I don't
think that's still possible or easy to do, so this basic
implementation seems like a valid start point.
Modify Dart.tmLanguage to enable symbol navigation

With this commit, it should now be possible to navigate to typedefs
and classes (as defined in Symbol List.tmPreferences). The syntax
file now defines additional scopes needed to locate symbols in
Dart source code.

Things to note about Sublime Text symbol navigation:

It appears that the `class` symbol is automatically detected by ST
and, what's more, the transformation for it defined in the
Symbol List.tmPreferences files seems to be ignored. That's why
I've commented it out until I figure out what's going on. It may
be that some rogue .tmPreferences file is overriding Dart's symbol
list.
move .tmPreferences file to Support/
Merge pull request #64 from guillermooo/symbols

Add ability to navigate to 'typedef' and 'class' symbols