Bug 421482 - UX: No context menu!
- extensionCommands.js cleanup
- Added caching to extensionCommandsUtils.createFileCommands(), all commands are now always created upon first call, only relevant commands are returned. Commands are only created and added to registry once.
- Cleaned up extensionCommandUtils.createAndPlaceFileCommandsExtension and made it place the commands into submenus by giving the Extensions and OpenWith groups titles
- Added excluded content types support to extensionCommands.js
- Made Command constructor (commands.js) set its own isEditor flag based on the passed in options rather than having extensionCommands set the property on the object from outside of the module

--Signed-off-by: Elijah El-Haddad <elijahe@ca.ibm.com>