Better members' encapsulation in ``EclipseSbtBuildManager``

Encapsulate members and added override indicators in
``EclipseSbtBuildManager``.
Deprecation warnings no longer accumulate

The reason why that used to happen is that the Eclipse markers attached to the
dependent sources recompiled by Sbt were not removed, and hence the deprecation
warnings (or any other kind of problem marker) were being accumulated after
each incremental build.

The fix consists in deleting all problem's marker attached to a unit when the
unit is about to be parsed. This should really be seen as a workaround until
Sbt provides a better interface for incremental compilation. In fact, ideally
Sbt should provide a notification mechanism to know what is the set of
compilation units that are about to be recompiled, **before** the actual
recompilation step is performed.  Unfortunately, that is not possible at the
moment, hence we are left with the current hack.

Huge thanks to @gkossakowski for suggesting this very effective workaround.

Fixed #1001595
Merge pull request #406 from dotta/issue/deprecated-warnings-accumulate-1001595

Issue/deprecated warnings accumulate 1001595
Scala IDE now builds again against Scala 2.11.0-SNAPSHOT

With the previous test code, one additional warning was reported
(something like _expression in statement position_). The fix was
to wrap the expression in a `println`.

Failure was introduced in commit SHA: 56e4328e736e5f3aa2e424f741cf80e92e3d1d2b
Merge pull request #409 from dotta/transient/fix-scala-211-build

Scala IDE now builds again against Scala 2.11.0-SNAPSHOT