Clean up .classpath files

Some .classpath files still contain references to SCALA_CONTAINER and
SCALA_COMPILER_CONTAINER which are not needed anymore. Furthermore
sdt.core explicitly references JavaSE-1.6 as its runtime environment
which shoudn't be included either because this doesn't allow a JRE
independent build.
Reformat old code.
Catch `JavaModelException` if a resource isn't there.

This code might be called for elements that were deleted. The original
code tried to be robust, but failed to treat checked exception `JavaModelException`.
Use the presentation compiler to check for existing types.

The original code relied on Java search, that can be slow and imprecise
for Scala types.
More robust handling of parse errors in formatter.

Return an empty mixup, rather than null.
Removed non-functional code for code generation.

Removed support for generating stubs for constructors
and inherited abstract methods:
* the UI doesn't show the section anymore
* ported constructor stubs to the presentation compiler, but
  disabled. Someone motivated could pick it up (works for simple cases).
* deleted code based on JDT type hierarchy for implementing
  inherited abstract methods.

This should fix JavaModelExceptions on class creation.

Fixed #1002006
Merge pull request #625 from dragos/issue/fix-new-wizard

Fix New Class Wizard JavaModelExceptions
Merge pull request #626 from sschaef/t-cleanup-classpath-files

Clean up .classpath files