[WIP] Project system

# Work In Progress

This branch is a work-in-progress. The pull request is being created for ongoing discussion of the project system *implementation* prior to incorporation in the project.

## Remaining work

* The branch includes many unnecessary features in the project system property pages, and/or items which do not make sense for Dart projects
* [ ] Need to disable the scanning, and only enable Dart analysis for non-**.dartproj** projects if/when a Dart file which is in that project gets opened (#38)
* Project templates
  * [ ] Identify reasonable project templates for Dart projects
  * [ ] Implement the project templates
* Item templates
  * [ ] Identify reasonable item templates for Dart projects
  * [ ] Implement the item templates