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.