Removed FIXME in `ScalaSourceFile` (Refactoring)

I thought the only way to get rid of the FIXME was to introduce a
`NullResponse`, which is why I believed it was a good idea to extract a pure
interface from `Response`. As the code demonstrates, it turns out I was wrong in
believing I needed a `NullResponse`.

As a side note, while I think it may still be a good idea to turn `Response`
into a pure interface, I realistically don't see a use-case.  Further, it turns
out that implementing the semantic of a `NullReponse` wouldn't be
straightforward, because the current implementation of `Response` is somewhat
incorrect in my opinion. The issue I see is that a `Response`'s answer shouldn't
change after it is `set`, however this is not enforced in the current
implementation (e.g., one could call `set` and aferward `raise`, and the answer
to `get` would be different depending if it's executed before or after the call
to `raise` - I would consider this a bug). Hence, it currently doesn't make much
sense to turn `Response` into a pure interface.
Do not use system properties to configure incremental compiler.

Since sbt 0.13.0 we have an official mechanism for configuring
incremental compiler: IncOptions class. The IDE has been partially migrated
to use that mechanism but one use of old mechanism (that relies on JVM
system properties) was left intact. The `incDebugProp` was used to enable
debugging of relations.

We switch to `IncOptions.relationsDebug` to control debugging of relations
and get rid of manipulating system properties.

This change makes IDE compatible with all sbt 0.13.x versions (including
0.13.2-M1).
Merge pull request #620 from gkossakowski/incDebug-property

Do not use system properties to configure incremental compiler.
SI-8151 Prepare for removal of -Yself-in-annots

Upstream, Scala will remove the `selfsym` field from `AnnotatedType`.

This commit removes its uses in Scala IDE:

  1. In debug printing code
  2. In a commented-out pattern match
Merge pull request #627 from retronym/topic/annotated-no-self-sym

SI-8151 Prepare for removal of -Yself-in-annots
Merge pull request #619 from dotta/wip/remove-fixme-in-scala-source-file

Removed FIXME in `ScalaSourceFile` (Refactoring)