Get filename from active document

Before it was always using the same filename
Wait 500 ms before calling isValid

Avoids calling isValid on every key press. Instead, it is only called if
key presses are distant by more than 500 ms.
Use start of selection as position

This way Rename is available when an entire identifier is selected
Set default reponse to OK

Pressing enter in refactoring dialog presses OK
Pull recent changes to fsharp-refactor