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.
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
More robust handling of parse errors in formatter.

Return an empty mixup, rather than null.