Clean the house in XWalkExtension(Instance)

- Updated the comments to be more on par with existing implementation;
- Added an overview at the top;
- Removed unused includes, forward declarations;
- Moved Callback types to XWalkExtensionInstance;
- Shuffled some functions to their appropriate places.

This patch shouldn't change code behavior.
Rename XWalkExternalContext to XWalkExternalInstance

The concept is now called Instance instead of Context. This allow us to
remove the typedef hack in XWalkExternalAdapter.
Rename xwalk_external_context.cc/h to xwalk_external_instance.cc/h

To match the class names. Patches are separated so the name change in
code is easier to review. This patch just renames the files and updates
references to it.