Add hierarchy handler stubs
Get the "Navigate->Type Hierarchy" menu item to display.
Add utils to:
 + find a class declaration from a position within the file.
 + find all top-level classes declared in a file.
 + find a HaxeFile node at the top of the PSI tree corresponding to an
   edit position within a source window.
Merge branch 'master' into ClassHierarchy
First stab at enabling the call hierarchy menu item.
Merge branch 'master' into ClassHierarchy