Always call the callback

They should know what to do with empty strings.
Fix a whitespace nit
Simplicy decision calls
Check for 'frozen' before invoking handlers
Make navigation requests async
Request a download URI async

Also fix some things related to file:/// and absolute path detection.
Make request handlers async

Also make WebKit2 use request_handler rather than the new mime_handler
for request handling.
Always append a destination uri
Make scheme handlers async
Dedent case blocks

It's unnecessary whitespace.
Block other handlers when frozen
The value can't be NULL
Handle new window actions as well
Results don't have newlines in them
Add a TODO item