Fix passing the wrong fileTypes type for showOpenDialog

When there's no restriction on the file types, an null should be
passed. The Extension expects an empty list in this case.
Add new Crosswalk Extension header

This is in preparation for converting Brackets to the new Crosswalk
C API.
Convert Brackets extension to the new Crosswalk C API

The code could be simplified because the Brackets Extension doesn't
keep any state, so no need to keep track of Extension Instances.
Remove header file for old Crosswalk C API