add try/catch to async extension init to fix #4404
refactor ExtensionManagerViewModel.

This is to prepare for further refactoring in support of consistent view
between multiple views.
handle more extension load error cases. add new unit tests.
rename init to initExtension
code review comments
fix jshint error for jasmine globals
Add drag and drop to open folder.
Merge branch 'master' into glenn/drag-and-drop-folder
Merge pull request #4405 from adobe/jasonsanjose/issue-4404

Rename auto-called extension init method to avoid collisions (fixing #4404).  Add try/catch to async extension init for robustness.
Use pre-computed length.
Merge pull request #4430 from adobe/glenn/drag-and-drop-folder

Add support for dragging and dropping a folder onto Brackets
Merge pull request #4426 from adobe/dangoor/registry-model

refactor ExtensionManagerViewModel.
minor code refactoring
minor code refactoring
Merge remote-tracking branch 'origin/master' into couzteau/fix_4238